Tire Pressures

[cbarge]

It is best to read on the tire to see what the manufacturer recommends.

Never go by the tire itself. Always go what the vehicle tire placard says.
Bias ply on my car is at 26 psi. I run radials at 32. An addition of 6 psi as per industry standards.
Car calls for 24? Do 30 with radials.
Its that simple.
Lost of posts making a mountain out of a molehill.
Our cars are pushing 50 plus years old and many in different stages of condition and/or wear.
Set the pressures to whatever you want that makes you feel comfortable and safe to drive.

Been in the biz since early 90's and been around C bodies all my life.

 

[Slap Stick]

Those were defective Firestones if I remember correctly. Most of the issues were in Texas and Ariz at high temperatures.

There were two Firestone/Ford debacles. One was the defective Firestone 500s, which were defective. I had a new car that had them and they were pure junk. That was the late 70s. The other was with the Explorers, which had to do with overloading and being undersized. A cousin was a tire factory rep then used to joke about the competitor and their junk tires. He had a purdy green loaded 69 383 Super Bee.

 

[Tire Whisperer]

Remember the Firestone/Ford Exploder tire debacle? It was due to pressure recommendations being far too low for the vehicle and the tire. It caused the tires to run hot and when cornering hard for the tire to fold under, causing the vehicle to overturn.
As mentioned earlier, they set pressures low on the big vehicles to promote a soft ride.

Having watched much of the federal inquiries into the Ford/Firestone deal, I was amazed that the Bridgestone/Firestone execs seemed to know so little about things! It was also revealed that Ford wanted 29psi for the recommended tire pressure and the tire engineers wanted 30psi. Ford won out. BUT 1psi difference wouild not cause a tire to fail, I suspect, especially when the tire psi is already in the "good range" NOR would that little difference result in a smoother ride

Just so you know, Ford selected 26psi for Explorer 1.0 as a cheap way to counter high center of gravity and rollover tendency of the first-gen Explorer.

A smooth ride was a benefit of the 26psi spec, but not the primary goal.

 

[MoPar~Man]

This tire shop in California has a good web page that talks about several different aspects of air pressure in cars and trucks.

Tires 101 on Air Pressure :: Souza's Tire Service

Here's one example:

==========
California passed a law in September 2010 concerning air pressure. It roughly states that: in order to reduce greenhouse emissions caused by underinflated tires, all auto shops are required to set the air pressure in any car they work on to the vehicle manufacturer's specifications (usually found on the door placard). That's all well and good, we want to decrease greenhouse emissions, but there's a problem, for easily more than half the vehicles we work on, the vehicle manufacturers specifications are either patently too low (for this area), or barely adequate. So in order to reduce greenhouse emissions caused by underinflated tires, we were required to underinflate them(!?). Fortunately, they soon realized this, and issued a clarification to the law that they would allow us to put more air in the tires if the customer requested it. We are still not allowed to go lower.
==========

There's a lot more, I won't quote the whole page, but I will quote this part:

============
One exception to this which is very common, is for oversize tires. Since, as stated before, weight capacity is a function of air pressure times air volume, you will often see larger truck tires primarily available in lower load ratings. A common example woud be an LT265/70R17 load range E and a LT285/70R17 in a load range D (there are E rated 285s but D they are the norm). The maximum inflation of the 265 in the E range is 80 psi and the maximum inflation of the 285 in the D is 65. However, due to the larger air volume of the 285, at maximum inflation, these two tires have the exact same weight capacity (3005), down to the pound! In this case, the tire pressure can be adjusted according to charts we have.
============

As I've stated before, tire size and load capacity and air pressure are interrelated.

I say, and someone prove me wrong, that the air pressure on the sticker on the door jamb is only correct when you have THE EXACT SIZE TIRE THAT CAME WITH THE CAR WHEN IT WAS NEW.

As we have 50-65 year-old cars that came with tire sizes that we can't even find exact manufacturer size data on (tell me the specs on a 8.25 x 14 tire) and many have put wider tires on our cars than they came with, changed the profile, not to mention the difference between ancient multi-ply bias vs modern radial, the air pressure numbers on the door stickers are worthless.

The car doesn't care what the air pressure in the tire is. As long as the rolling diameter matches what the speedo / odo is calibrated for, the car doesn't care. BUT THE TIRE CARES. An under-inflated tire is a not a happy tire.

There are probably tables somewhere that for a given tire-size and load will give you the "correct" air pressure you should have in the tire to allow the tire to have the shape it was designed to have while rolling. Such a table might very well be tire brand / model dependent, factoring in specific construction details and rubber compound formulas.

Car companies no doubt consult tire makers (ie the tire maker chosen to supply the OEM tires when the cars are sold new) so the air pressure on the door sticker is based on that. No doubt today the air-pressure number is heavily weighted towards fuel economy, just as we know 50+ years ago it was weighted towards "ride comfort".

 

[Tire Whisperer]

This tire shop in California has a good web page that talks about several different aspects of air pressure in cars and trucks.

http://[URL][URL][URL][URL]https://www.souzastireservice.com/Tires-101/Air-Pressure

[/URL][/URL][/URL][/URL]

Here's one example:

==========
California passed a law in September 2010 concerning air pressure. It roughly states that: in order to reduce greenhouse emissions caused by underinflated tires, all auto shops are required to set the air pressure in any car they work on to the vehicle manufacturer's specifications (usually found on the door placard). That's all well and good, we want to decrease greenhouse emissions, but there's a problem, for easily more than half the vehicles we work on, the vehicle manufacturers specifications are either patently too low (for this area), or barely adequate. So in order to reduce greenhouse emissions caused by underinflated tires, we were required to underinflate them(!?). Fortunately, they soon realized this, and issued a clarification to the law that they would allow us to put more air in the tires if the customer requested it. We are still not allowed to go lower.
==========

There's a lot more, I won't quote the whole page, but I will quote this part:

============
One exception to this which is very common, is for oversize tires. Since, as stated before, weight capacity is a function of air pressure times air volume, you will often see larger truck tires primarily available in lower load ratings. A common example woud be an LT265/70R17 load range E and a LT285/70R17 in a load range D (there are E rated 285s but D they are the norm). The maximum inflation of the 265 in the E range is 80 psi and the maximum inflation of the 285 in the D is 65. However, due to the larger air volume of the 285, at maximum inflation, these two tires have the exact same weight capacity (3005), down to the pound! In this case, the tire pressure can be adjusted according to charts we have.
============

As I've stated before, tire size and load capacity and air pressure are interrelated.

I say, and someone prove me wrong, that the air pressure on the sticker on the door jamb is only correct when you have THE EXACT SIZE TIRE THAT CAME WITH THE CAR WHEN IT WAS NEW.

As we have 50-65 year-old cars that came with tire sizes that we can't even find exact manufacturer size data on (tell me the specs on a 8.25 x 14 tire) and many have put wider tires on our cars than they came with, changed the profile, not to mention the difference between ancient multi-ply bias vs modern radial, the air pressure numbers on the door stickers are worthless.

The car doesn't care what the air pressure in the tire is. As long as the rolling diameter matches what the speedo / odo is calibrated for, the car doesn't care. BUT THE TIRE CARES. An under-inflated tire is a not a happy tire.

There are probably tables somewhere that for a given tire-size and load will give you the "correct" air pressure you should have in the tire to allow the tire to have the shape it was designed to have while rolling. Such a table might very well be tire brand / model dependent, factoring in specific construction details and rubber compound formulas.

Car companies no doubt consult tire makers (ie the tire maker chosen to supply the OEM tires when the cars are sold new) so the air pressure on the door sticker is based on that. No doubt today the air-pressure number is heavily weighted towards fuel economy, just as we know 50+ years ago it was weighted towards "ride comfort".

We shall have to respectfully disagree then.

Using relatively modern examples, and both vehicle and tire documentation to prove it, I demonstrated that in most cases, the door placard cold tire pressures are sufficient to carry the gross load (the maximum) of the car, plus a ten to twenty percent safety margin.

So when you are driving alone to work, just you in the car with your laptop or lunch bag, and half a tank of gas (you'll stop to fill up this evening after work), your tires are far far more than capable of supporting that load! Again, zero reason to increase cold tire settings, or change them up or down, unless you plus- or -minus-sized your tires, at which case a different cold pressure may apply.

On my 2010 Honda Accord, EX trim, the OE 225/55R17 size tires, at 32psi cold per Honda, support, respectively, 111% and 129% of front gross-axle and rear gross-axle weight. That is, an average of 120 percent of gross load: full gas tank, driver plus four passengers, luggage, and the family dog.

Now who carries that gross load every day? Not many folks I know, unless they do rideshare business. The point is, at 32psi, the tires on my car are not, as you think, "under-inflated", or chosen "for a softer ride".

I'm presenting facts and figures here, science, not blowing smoke out the top of my head.

If I decide to maintain the tires in that example Accord at any pressure, higher - or lower - than what's on that door placard, that is only my opinion and personal preference to do so.

My car sets the load limits, not the tires.

Sure, my tires can carry, at 36psi, a much higher load than they could at 32psi - but:....

Is the VEHICLE - in this case my Honda - rated to carry that higher load?

hmmmmm...

Makes you rethink this whole over-inflation addiction the whole country seems to have now, doesn't it.

The only, and theoretical at that, advantage for me to maintain my Accord tires at 36 or 40psi is for hyper-miling, or attempting to get the most miles out of a full tank, or on an EV, the most miles per charge.

Other than that, running pressures more than 1psi(to account for seasonal temperature swings) above the door placard pressure is simply opinion and preference.

Yes, I do know there is a big exception to this:

Converting an older car from bias to radial ply tires. And that's why finding out, or knowing, the front /rear gross axle weight is is critical when determining what cold tire pressures to run.

I get the same refutation as from you, on other automotive discussion groups and newsgroups, about "door sticker pressures being too low", and how "the max pressure on the tire is needed for max load capacity", etc, and frankly, it almost makes my blood boil. So I'll stop now, and you all can do what you want with it.

 

[MoPar~Man]

We shall have to respectfully disagree then.

Just answer this one question:

Does the air pressure on the door sticker apply to a specific size tire?

I suppose the answer is obvious if the tire size is also printed on the sticker.

But what about our 50-60 year old Chrysler cars?

You are here, Tire Wisperer, in this forum that is centered around these old cars, where we have to deal with tires as they exist in the market today, radial tires no less, and we make many decisions on tire sizes, sometimes based on how they look, usually not knowing how exactly they differ from the tire that came on the car when it was new and the corresponding door sticker to match.

So, tire wisperer, does it matter if I have a 205-70-15 vs 235-60-15 tire on my Chrysler? They have the same diameter, within 1% of each other.

The all-knowing door sticker will still tell me that the correct air pressure is 28 psi. The sticker doesn't know what tire size I have. All it knows is that single number. 28 psi. What say you about that?

 

[Tire Whisperer]

Just answer this one question:

Does the air pressure on the door sticker apply to a specific size tire?

I suppose the answer is obvious if the tire size is also printed on the sticker.

But what about our 50-60 year old Chrysler cars?

You are here, Tire Wisperer, in this forum that is centered around these old cars, where we have to deal with tires as they exist in the market today, radial tires no less, and we make many decisions on tire sizes, sometimes based on how they look, usually not knowing how exactly they differ from the tire that came on the car when it was new and the corresponding door sticker to match.

So, tire wisperer, does it matter if I have a 205-70-15 vs 235-60-15 tire on my Chrysler? They have the same diameter, within 1% of each other.

The all-knowing door sticker will still tell me that the correct air pressure is 28 psi. The sticker doesn't know what tire size I have. All it knows is that single number. 28 psi. What say you about that?

Most modern, and some vintage, vehicle frame tire placards specify OE tire size yes. And the pressure on it applies to that size.

But when you change tire and or wheel size, or tire construction type, conversion tables exist for you to find the correct pressure, given the plus- or minus-size or construction in question.

In general, going to a wider, lower profile tire on a larger rim usually means a 1-2psi increase in cold tire pressure, to ensure the wider tread maintains even contact across its width.

But in simpler cases like mine: replacing OE tires size-for-size, there is zero reason to change the cold pressure from what on the door frame. There is also a lot of ignorance out there:

I've been told that since I no longer have the factory Michelin Pilots on my Honda, and now have Pirellis, that the "door placard pressures are invalid". Those are idiots talking, pay them no mind!

The 28psi you mentioned on your older car was fine for the OE bias ply tires it likely came with, new.

By the very nature of radial construction and performance dynamics, a higher pressure would be required to carry the same load and maintain similar performance characteristics as with the old bias ply tires.

All I'm saying is, don't just pull that new pressure number out of a hat, or assume that the 44 or 51psi max cold pressure on the replacement radial tire sidewall is the correct pressure for your specific vintage car.

By now you probably get that I am far more concerned with over-inflation of tires than with underinflation, although there's plenty of that out there also.

 

[CBODY67]

In looking at your back-and-forth, TW is advocating an OEM orientation of 20% margin between total vehicle weight and tire capacity (on each axle), as MM is asking about "older vehicles" and what is correct for them. Tire construction has NO influence on these things. Tire construction affects the HANDLING characteristics of the tires, not load carrying, from what I have seen.

On a vehicle built since the 1980s, the axle weight has been on the VIN sticker on the driver's door shell, in plain sight. In earlier times, it was either on the glove box door or in the owner's manual, starting in about 1968 or so.

We know that as many of those labels/stickers were subject to "atmosphere", they could get dirty. Some cleaning agents can and did wipe the printing off of the stickers. So what to do then? With no numbers? What "work arounds" can be good and also legal, should something unforeseen happen and questions get asked?

In a prior post, I mentioned finding the appropriate Chrysler factory service manual (1955 to 1974) at a particular website. WE know about that website, but what about similar locations for GM and Ford, or other vehicle brands? Something that can be trusted to be accurate, at least for the base equipment vehicle tire size, that employees can be referred to.

Might a website as www.TireRack.com or www.tires.com (Discount Tire) website have a "shop by vehicle" option on them? I'm not sure, but I believe they do. When I go into the TireRack website, I usually use a tire brand and model orientation.

Somebody who has more knowledge about older cars might also know "what looks right" on the cars, with respect to physical sizing. Taking away any "what fills the wheel well better" orientations in the process. It would need to be presumed that whomever sold the tires put something on the car which would adequately support the weight at 30psi inflation pressure, with a max inflation pressure of at least 35psi for passenger vehicles.

On ALL of the 1990s cars which came with P225/60R-16 tires, which was a very common size in those times, they all had 30psi on the tire minimum size and pressure decal. No matter the OEM of the vehicle.

Now, if a customer comes in with a '64 Impala and 14x4" wire wheels and equally small diameter tires, it can reasonably be presumed those are NOT factory wheels or tires. In which case, what's on the tire sidewall (max inflation pressure) would need to be consulted. Not unlike a late model pickup with "different wheels and tires". In those cases, I would not have any issues with following what was on the tire sidewall as to max inflation pressure. In either case, I somewhat doubt the customer would complain about following that decision rule, unless they can supply other documentation about that.

Absent of any OEM documentation on the car body, let the car with normal tire sizes on it leave the shop with 33psi cold inflation pressure. Only problem is that if the car drove to get to the shop, the tires will not be "cold" and have a higher inflation pressure in them. In that case, check them for equality to the highest pressure of the bunch. Knowing they will be closer to the cold inflation pressure in the morning.

The issue of ambient temperature has not been mentioned! In high school physics class, me and my trusty slide rule determined that for every 5 degrees F of ambient temperature change, inflation pressure will vary by 1psi of cold inflation pressure. Which can mean that a tire inflated to 33psi at an ambient temperature of 85 degrees F will need the pressure "seasonally adjusted" when the temperature falls to 50 degrees F. Same thing in reverse.

IF one reads and understands the "Specs" chart for each tire model that TireRack sells, it can become apparent that, even going back to look at the old tire sidewalls for max inflation pressure and related load carrying capacity, it can be determined that a 8.25x14, G78x14, P215/75R-14, and P225/70R-15 tires all have a max capacity of right at 1750lbs at 35psi. Which comes out to 7000lbs total tire capacity. On a car that can weigh 4200lbs "ready to roll". Which is about 2000lbs past the 120% of vehicle weight previously mentioned. Adding 2000lbs of occupants and luggage, results in a vehicle weight of 6200lbs, which is 440lbs past the 120% orientation. Personally, I don't think I would want to be in a car with that kind of loading, period! Not from a weight orientation, but from a personal comfort orientation!

I suspect TW and MM might have points to discuss about my above proposal, based in reality and physics. Also which should be legally defensible in a court of law, if needed. BTAIM

Y'all enjoy!
CBODY67

 

[CBODY67]

As to the Ford-Firestone proceedings I watched on C-SPAN, there were several points I noticed. Looking at the tire capacity at 35psi and the weight of the vehicles in question, IF the tires were at 35psi, the rear suspension of the vehicle would have been on the bump stops at that weight. Which would then mean the tires were nowhere near that cold inflation pressure prior to the incident.

Even if the tires had been at the slightly-lower Ford-advocated inflation pressure, after the vehicle drives for 10 miles, the tire flex will raise the inflation pressure by several pounds. Moreso if the tires started out at a lower pressure. Even with radial tires.

A complicating factor could have been that any lessons learned by Firestone in their "721 Radial" tread separation issues, the production people who were employed then knew how to make the computerized equipment work best, but when those people retired, that knowledge was lost and not replaced to train the new employes with. Probably similar with the chemists who designed the rubber mix and how the rubbers in the carcass and tread areas bonded and interacted. So, prior failures were repeated, possibly. When they did their initial work, things were designed to be very good, but something happened later on that tended to compromise that work, I suspect.

The other thing I noticed was the displayed lack of knowledge of the products which Bridgestone/Firestone produced by their highest executives. Apparently, the "tire business" they knew was more related to sales than engineering, and side issues thereof?

It appeared to me that there was plenty of blame to go around, on several levels of things. It's unfortunate that lives were lost in these situations.

Respectfully,
CBODY67

 

[Tire Whisperer]

Somebody who has more knowledge about older cars might also know "what looks right" on the cars, with respect to physical sizing. Taking away any "what fills the wheel well better" orientations in the process.

With a handle like Tire Whisperer, I actually never concern myself with how something looks when it has to do with the four items interacting between my car and the road.

This trend toward putting wider and wider tires on rims big enough for a Boeing 737, and adding spacers besides, so that the wheels protrude slightly out from the fenders, just make me chuckle at the juvenility of it all.

it can be determined that a 8.25x14, G78x14, P215/75R-14, and P225/70R-15 tires all have a max capacity of right at 1750lbs at 35psi. Which comes out to 7000lbs total tire capacity. On a car that can weigh 4200lbs "ready to roll". Which is about 2000lbs past the 120% of vehicle weight previously mentioned.

7,000lbs cap at 35psi cold / 4,200 curb weight("ready to roll") = 1.66 or 166% safety margin. Not down to 1.20 yet.



"Adding 2000lbs of occupants and luggage, results in a vehicle weight of 6200lbs, which is 440lbs past the 120% orientation."

First of all, that 2,000lb max load is something one might see for a light truck or pickup, or a van.

Let's pick a more reasonable typical max load for a sedan or wagon, like 950lbs: (I've seen as low as 800lbs on vintage car placards, back when there was hardly a domestic obesity crisis)

7,000 / (4,200 curb + 950lbs of pax & luggage = 5,100) = 1.35 or 135%.

As I stated before, anywhere between 110-120% of gross(fully occupied, fueled, plus the family Doberman) is the industry norm.

When I ran the same calculations for the few 1970s cars where gross axle(GAWR) was known, the margins were much narrower - 103-105 percent...!

 

[MoPar~Man]

The following selected text (below) is taken from here:

Tire Size 101

Tire Size 101 - Mecum Auctions - Discover expert insights, fascinating stories, auction news and the latest trends in the collector car, vintage motorcycle and Road Art industries.

www.mecum.com

Some useful nuggets of info that I've never seen anywhere else:

- Numeric sizing leads off with the tire’s approximate section width, listed in inches (ie not tread width)

- A tire size that ends with a zero, such as 6.70-15 featured an approximately 90% aspect ratio.

- A bias-ply tire ending in the number five, such as 6.95-14 featured an 80% to 84% aspect ratio.

8.25-14 would therefore have been an 80% to 84% aspect ratio. The wide latitude in aspect ratio is not particularly helpful here. 8.25-14 would theoretically correspond with 209R80/14 in P-metric size assuming 80% aspect ratio. The closest P-metric tire would be 215/75R14 and would be 1.8% smaller in diameter and be off by 1 mph at 60 mph (13 revs/mile).

- Alphanumeric tire sizing was standardized in 1967, but I see 1967 Chrysler/Dodge service manuals still using numeric sizing so presumably this changed for 1968 models.

- The letter in the alphanumeric system did not directly code for tire size - it codes for tire load capacity. The 2 digits after the number does indeed code for aspect ratio. The "78" aspect ratio was apparently the first or at least the most common, smaller ratios like 70 came later. This would seem to make it somewhat murky if one tire maker's G78-14 was or was not the same dimensions as another maker even if their load ratings were exactly the same.

=============================

Tire Size 101
The evolution of tire sizing

There are three common types of tire sizing from the 1920s through the modern era: numeric, alphanumeric and P-metric. Uncommon forms of tire sizing are used for high-pressure tires (Brass Era), racing tires, vintage metric tires and certain types of off-road truck tires. Let’s cover the three most common tire sizing styles.

Numeric sizing is most common on bias-ply tires manufactured from the late 1920s through the late 1960s. This started with the tires that were marketed as Balloon tires, due to their lower air pressure and soft ride, compared to the earlier high-pressure tires of the 1900s and 1910s. Numeric sizing leads off with the tire’s approximate section width, listed in inches, and then features the rim diameter (ex. 6.70-15 or 8.00-14).

From the 1930s onward, American cars gradually transitioned into smaller wheel diameters, while increasing tire and wheel width. Any tire size that ends with a zero, such as 6.70-15 or 7.10-15, featured an approximately 90% aspect ratio. Any bias-ply tire ending in the number five, such as 6.95-14, 7.75-15, etc., featured an 80% to 84% aspect ratio. We continue to use numeric sizing in most of our bias-ply tires, as well as our bias-look radials. For radial tires, we simply add the letter “R” in place of the hyphen (ex. 6.70R15).

Alphanumeric sizing became common in 1967 and was the industry standard only a few years later. This type of sizing takes the guess work out of the aspect ratio, but it creates some guess work on the tire’s overall width. The size starts with a letter, which is the tire’s load range. The higher the letter, the larger the tire. A78-13 is an example of a very small tire for an economy car, and L78-15 is an example of a large tire for a Cadillac. The two-digit number after the load range rating is the aspect ratio. The introduction of alphanumeric tires also saw the introduction of smaller aspect ratios, and shorter sidewalls. The 78 series was common, but 70 series and 60 series came shortly after.

The muscle car era saw creative usage of alphanumeric tires, including the Firestone Wide Oval. It was available in redline originally and then later in raised white letter. Popular sizes for the Firestone Wide Oval included F70-14 and G70-15. Aftermarket tire companies, such as Pro Trac tires, offered custom alphanumeric sizes, such as N50-15, an enormous tire that was typically used on the back of modified muscle cars. Radial tires were introduced to American markets in the late 1960s and became original equipment offerings in the mid-1970s. The alphanumeric sizing remained for a short while, but an “R” was added after the load rating (ex. LR78-15).

The most current form of tire sizing is P-Metric. This sizing nomenclature can be confusing because the section width is listed in millimeters, the aspect ratio is listed as a percentage, and the wheel diameter is most often listed in inches. For example, a P205/75R15 tire features a section width of 205 millimeters and a sidewall that is 75% of that width. The “R” tells us it’s a radial, and the 15 indicates a 15-inch wheel diameter. This has been the industry standard in the United States since the 1970s, but the idea for this metric size came a couple decades earlier in Europe. British sports cars featured tires such as 165R15, dating back to the 1950s and 1960s. You’ll notice that there is no aspect ratio designation in this example. When a size is listed with only the section width and rim diameter, you are to assume that it features an 80% aspect ratio.

P-Metric radial tires are a common replacement for original bias-ply tires. In the case of 1950s classics, we most often suggest a 205/75R15 for cars that originally came with a 6.70-15 bias-ply tire. We also use 205/75R15 radials as a cross reference to original 7.75-15 Corvette tires.

 

[CBODY67]

Yep, my predictions were accurate, more "conversations".

TW, after I dkd the original calculations on tire reserve capacity on a 4200 lb "ready to roll" vehicle, I then thought, "That's an empty vehicle sitting at the curb". Then, in a typical 1965 Plymouth Fury III, that vehicle is sized and designed to hold SIX full-size adults. If we presume that they could, at the time, weigh up to 250lbs each, SIX of them (including the driver) would weigh 1500 lbs total. Add another 500 lbs of luggage, and there is, suddenly 2000 lbs added to the basic vehicle. Yep, it would probably be bottoming out on each medium-sized bump/dip, but the springs would be supporting the total car weight, as would the tires at 35psi cold inflation pressure.

In reality, most full-grown adults back then could be more like 200-225lbs, but those possibilities COULD exist. Replace the adults with high school age kids, about 75-100 lbs lighter. No problem. Even better if they "travelled light".

These figures reflect "Vehicle Max Load" capacity as stated by the OEM. In the Owner's Manual and Factory Service Manual. In reality, as mentioned, most vehicles usually carried 2-4 occupants of vaired weights, back then, with probably 100lbs of stuff in the trunk. Making the tire reserve capacity lower so lower inflation pressures could easily be used. It ALSO seemed that MORE owners AND THE PROPLE WHO TOOK CARE OF THEIR CARS (think service station attendants!!!) knew abotu inflation pressures and intended use of the vehicle. "Soft ride" and local driving (wiht poorer tire life) was 24psi, As my '70 Monaco manuals stated, +4psi for 70mph driving, and up to 32psi for max vehicle loads. NO problems, back then. People knew these things, or could read them IF they knew where to look.

60 years later, the friendly gasoline-installers have vanished. No motor oil or tire checks with each tank of fuel. The people whom we kidded about only knowing how to put gas in their cars (and little es=lse) have seemed to proliferate. Owner's manuals still exist in paper forums, but also in electronic forms, too. Rather than 100 pages, most now approach 400 pages to educate modern drivers about "How to Drive" and such. Tending to reflect the modern world where high school summers are not taken up with "Driver's Education" classes and getting a driver's license AFTER semi-trained instructors taught the course and then over-saw "on-road learning".

Here's my suggestion, TW, with your industry connections, I would suggest you seek out the document published by the old (paraphrased) "Tire and Rim Manufacturer's Association", published annually and sent to tire sellers each year. I personally saw them at a local tire retailer we patronized. I know they existed in the middle 1960s, and probably prior to that. Each year's publication was over 200 pages of "industry information and orientations" a good tire dealer could consult, if needed, as a valuable information source. In some respects, prior information was repeated each year, BUT also updated for any new information with each new model year.

I also know it was available when the then-new P-Metric tires appeared. With their orientation of basing tire replacement sizing on weight-carrying capabilities with the higher 35psi rather than on physical circumference (which would maintain speedometer/odometer accuracy).

MM, you probably already know how to acess the Chrysler Factory Service Manual archives at www.mymopar.com . Looking at those service manuals as to tire concerns might be helpful in your quests for knowledge. Then digging into the magazine archives at the Automotive History Preservation Society's website for magazines published in the 1960s which had tire articles in them. The website might not be the easiest to navigate, but the information in there can be worth the effort. I'm not sure all of the CAR LIFE magazines from the 1968 era, with ORIGINAL explanations of new tire advances have been downloaded, though. Or some of the good articles in "Popular Science" magazine (which might be elsewhere on the Internet).

As to the later-1960s, then-new, Alpha-Numeric tire sizing, in about 1975 I was given a copy of a "CARS" magazine which had a complete list of all of the possible new sizes AND specs on such. It has been carefully-archived in my magazine collection, as have all of the CAR LIFE magazines I have mentioned.

These are my recommendations of "where to look" for historic tire information related to sizing and inflration pressures. Should there be any doubts about modern sizes and their ultimate per tire weight carrying capabilities, www.TireRack.com has a huge amount of tire-brand-model-size information, in a somewhat easy-to-navigate format.

Enjoy your quests for knowledge,
CBODY67

 

[Tire Whisperer]

Tire and Rim Manufacturer's Association", published annually and sent to tire sellers each year. I personally saw them at a local tire retailer we patronized. I know they existed in the middle 1960s, and probably prior to that. Each year's publication was over 200 pages of "industry information and orientations" a good tire dealer could consult, if needed, as a valuable information source.

I'm quite familiar with that organization.

Most of their information is probably priceless, which is why I refuse to pay for information/ documents that should be available freely to both service professionals and the general driving public.

 

[MoPar~Man]

This is a fascinating document, which I've just stumbled upon. It goes into extreme detail into all the various aspects of tire sizing and loading and even marketing that have been discussed in this thread.

==============
Report of the Federal Trade Commission on the tire safety aspects of the Commission's proceeding relating to the marketing of automobile tires

Report of the Federal Trade Commission on the tire safety aspects of the Commission's proceeding relating to the marketing of automobile tires

===============

This is a report that contains details of hearings (congressional?) held during 1964 and 1965 on the topic of tire safety and covers all sorts of aspects of passenger car tires. This was apparently a time of great changes in the auto world - the interstate highways and high-speed long distance driving was emerging and there seemed to be enough tire failures to demand action by regulators and politicians.

If you wanted to know the mind of the tire world, as it existed in the early 1960's, I can't imagine a better document. The aftermath of these interviews and testimonies no doubt shaped the tire industry and their products during the period 1965-1970.

 

[Tire Whisperer]

This is a fascinating document, which I've just stumbled upon. It goes into extreme detail into all the various aspects of tire sizing and loading and even marketing that have been discussed in this thread.

And just as fascinating a container you loaded that into for our perusal - that Babel thing!

Could you if possible please load some tire load and inflation tables into it? For both Bias and Radial if possible please?

 

[Jakter]

What's the best valve stem caps to use?
Which air is better, from my Ingersoll Rand compressor or my Craftsman compressor?

 

[MoPar~Man]

And just as fascinating a container you loaded that into for our perusal - that Babel thing!

Could you if possible please load some tire load and inflation tables into it? For both Bias and Radial if possible please?

I'm not sure I understand your question. There are dozens of pages of tables in this 484-page document showing various tire-related numbers.

What I'm seeing now, only after reading a few dozen pages and skipping through the document, is that they appear to have not had indepth focus on tire air pressure. The role that air pressure performs to give the tire it's load-carrying capacity, the role that air pressure provides to give the tire the service life it needs without suffering catastrophic failure, the concept of a "safe" air pressure for a given tire, or whether a variable air pressure specification is desired or expected (or is technically realistic) by auto makers to satisfy their needs for ride quality from a vehicle sales / marketing perspective.

An example of the lack of thought into air pressure can be found in this document (starting at page 195):

======================
Page #195

This proposed specification dated April 26, 1965 ZZ-T-C0381k as not been approved and is subject to modification. NOT USE FOR PROCUREMENT PURPOSES Int. Rev. of ZZ-T-00381) July 13, 1959 and Fed. Spec. ZZ-T-3811 August 6, 1957

PROPOSED INTERIM FEDERAL SPECIFICATION TIRE, PNEUMATIC, VEHICULAR (HIGHWAY) (preamble to be added)

1. SCOPE AND CLASSIFICATION
1.1 Scope. This specification covers pneumatic tires, both tube and tubeless -ype for mounting on passenger cars, station wagons, trucks, buses and similar ehicles operated on public roads and highways.
1.2 Classification. Tires covered by this specification shall be furnished n the following groups, sizes and ply ratings, as specified (see 6.2).
1.2.1 Groups 1 Passenger car (tube type) 3 - Truck-bus (tube type ) la - Passenger car (tubeless) ЗА - Truck-bus (tubeless) 2 - Light truck (tube type) 4 Motorcycle 2A - Light truck (tubeless)
1.2.2 Sizes and ply ratings. Tires shall be of the sizes and ply ratings listed n Appendix A as specified (see 6.2).
2. APPLICABLE SPECIFICATIONS, STANDARDS AND OTHER PUBLICATIONS
2.1 (to be inserted)
3. REQUIREMENTS
3.1 Certification. Tires offered under this specification shall be certified by the contractor to have successfully completed the laboratory tests (see 4.4.2), and he road test (see 4.4.4). FSC 2610

Page #199

3.8 Identification marking. Each tire shall be branded, molded or have permanently affixed in an unobstructed location on the sidewall the following information:

1. Manufacturer's name, brand name or trademark
2. Nominal size 3. Ply or ply rating
4. Serial number
5. "Tubeless", when applicable

=======================

Note that air pressure or even design load are not listed as the specs that tires must show under this proposal. Manufacture date is also not contemplated. We know that tires today and for many years have had those markings, so there are probably more of these sort of federal documents and hearings in later years that have revisited the issue of vehicle tires and have added those requirements.

This roughly 500 page document is focused exclusively on tire safety and forcing the industry to adopt testing and measurement and marking methods to deliver to the public a reliable and uniform product from that point of view.

The concept of tire "ply" is frequently mentioned throughout the document. It is seen as perhaps the most important or key aspect of a tire second only to tire size. Even tire tread depth seems to take a back seat, I have not come across it's mention in any regard.

I have come across other documents from the early 1970's where police car tire failures are discussed. Police agencies were keen on switching to radial tires because of their longer service life and better handling vs biased tires, but they were finding that these early radial tires were suffering blowouts during high-speed pursuits.

 

[CBODY67]

This is a fascinating document, which I've just stumbled upon. It goes into extreme detail into all the various aspects of tire sizing and loading and even marketing that have been discussed in this thread.

==============
Report of the Federal Trade Commission on the tire safety aspects of the Commission's proceeding relating to the marketing of automobile tires

http://[URL][URL]https://babel.hathitrust.org/cgi/pt?id=uc1.c025632814&seq=1

[/URL][/URL]

===============

This is a report that contains details of hearings (congressional?) held during 1964 and 1965 on the topic of tire safety and covers all sorts of aspects of passenger car tires. This was apparently a time of great changes in the auto world - the interstate highways and high-speed long distance driving was emerging and there seemed to be enough tire failures to demand action by regulators and politicians.

If you wanted to know the mind of the tire world, as it existed in the early 1960's, I can't imagine a better document. The aftermath of these interviews and testimonies no doubt shaped the tire industry and their products during the period 1965-1970.

This is an outstanding find! It details several things which later led to the alpha-numeric tire sizes which appeared in the later 1960s. Plus minimum testing/validation standards for all tires sold in the USA, by anybody.

This document also indicates that "self-policing industry standards" was not enough to keep things where they needed to be. As the "standards" used could vary from agency to agency, tire company to tire company, or even between government agencies. It should also be noted that the "General Services Administration" was the "central purchasing agent" of the USA government. Getting approval from them could be essential to a company's longevity, sometimes. Although some of their observed standards were "good enough for government work" (think "value for price paid", most probably), but might not be the highest standards available (at higher purchase prices).

The document also spotlights the fallacy of "letting the states police these things", rather than ONE government agency which would set standards, monitor performance, and possibly administer penalties. Having a patch-work of state standards could be a regulatory nightmare for companies to navigate, by observation. As things later evolved, the tire companies did their testing to federal specs and reported their findings in the areas of treadwear and stopping performance, resulting in the numbers and letter designations cast into the tire sidewalls, to be used by consumers in their tire purchases and performance comparisons. The consumer might not know what each of those ratings means, as to ultimate performance, but at least they know that a minimum level of standards have been met.

I would recommend that anybody interested in current tire standards might read this document, noticing the various orientations of the tire companies (which had their own investments in R & D and marketing to protect) to these proposals. Not to gloss over how the bulk of them used the word "safe" (variations thereof or allusions to) in their tire names and marketing publications.

It should also be noted that the vast majority of tires sold in the earlier-1960s time frame in the USA were bias-ply tires, using "cord" materials of rayon, nylon, and later polyester yarns. Radial-ply tires started to appear in 1967, from UniRoyal and BFG, touting better wet weather performance and higher-speed performance (remember the steel-masked law enforcement officer in the BFG print ads talking about how he could catch more speeders on his car's BFG radials?). Michelin and other European brands were in a niche market of car enthusiasts or were OEM on some of those imports, at that time, yet their market shares were starting to happen. As a higher price point. Until Ford started to use Michelins as standard or optional equipment in 1969, Michelin was a "high-priced alternative".

Thanks, @MoPar~Man for digging this document out of history and linking to it here!

CBODY67

 

[MoPar~Man]

Here's an interesting nugget, also courtesy of hathitrust.

It's a document from 1978 that references the tire hearings and testimony from 1966. From the point of view of analyzing the effectiveness of the effort, the cost/benefit of the excercise from a social and economic perspective.

The Uniform Tire Grading System:
A Case study of the government regulatory process

Here's part of the preface:

==========
PREFACE

The National Center for Productivity and Quality of Working Life is charged with identifying those regulations which adversely affect our Nation's rates of productivity growth. It has sought to identify the circumstances under which our current regulatory mechanisms allocate resources to meet social and economic goals and to determine if that allocation process is productive. This study was undertaken as an attempt to trace the complex interactions of government, industry, public interest groups, consumers, and the legal system in resolving a particular set of problems concerning automobile tires.

We believe that it can help illuminate the critical decision points that occur throughout our regulatory system and thereby assist the Nation in devising improved methods of problem-solving. The issue selected, tire quality grading, was chosen because it represented an apparent failure of the regulatory mechanism to resolve the problems presented to it. As of this writing, more than 11 years have elapsed since the law requiring a uniform tire quality grading standard was passed and the standards still do not exist.

Millions of dollars have been spent in the process, thousands of man-hours have been expended by the government and the private corporations, and to no avail. The case study examines the history of this attempt to regulate. The Center's interest is in the process and not with the performance of any particular group or individual. Our observations on the events outlined in the study are listed below. We feel some important questions have been raised but we have no desire to isolate "blame." The responsibilities for solving problems of national interest are equally shared by all participants in the process. Where the system is non- productive, it is the system that must be altered to improve its ability to generate useful results.

Observations: (1) It is interesting, and perhaps important, to note that two distinct problems concerning tires were raised before Congress at the same time (1964). One was the safety of tires; the other was the ability of consumers to compare the quality of tires. By 1968 the tire safety problem was resolved. Nearly 10 years later, the quality comparison problem remains. Involved are precisely the same mix of participants.

The uniform tire quality grading system : a case study of the government regulatory process

============

This is only 88 pages, but the first dozen or so are very interesting.

 

[CBODY67]

Tire "quality" will always be hard to measure in a consistent manner. In the 1960s, the orientation of "buying quality" meant buying "good brands" (which were all long-time USA-owned and manufactured brands at that time). Unfortunately, "price" was not always a good indicator of such, as one OEM brand was priced a good bit above other brands, by observation.

In those times, we were blessed with a long-time family friend who was in the tire and sporting good business in town. He sold tires to a bulk of people in town who did not go to the "main line" tire stores, although he also sold BFG tires. He judged "goodness" by the brand by the number of tires that did not have issues related to the tire itself. Dad trusted him and so did lots of other people. As a result, lots of the tires he sold were tires made by the big companies but under another name. They always held up well for us, except for one brand in the middle 1970s. No blow-out or similar, just tread wear issues for no car-related reasons.

Now, I should also mention that tires in those decades, for us, had no issues going to over 30K miles (in what I'd consider "normal use"). I made sure the tire pressures were always around 30psi cold and the Chryslers had no problems "maintaining alignment" for us. Compare that to the 92K I could regularly get from the BFG Radial TAs on my '77 Camaro LT. Or similar mileages from the OEM version used on smaller GM cars back then.

The one set of BFG Silvertown Belteds we put on the '66 Newport Town Sedan got off to a flaky start as one was visibly and operationally "out of round", with the two mold halves not being concentric with each other. A new tire fixed that. I considered that "a percentage deal".

Through other car service people, I learned about the "high quality" of the old Atlas Plycron bias-ply tires. Where ALL of them would balance with only 1/4 ounce of balance weight. If it took more, "something was wrong", by observation. With decent care, no issues with getting 30K miles before it was replacement time. Then people got another set of them from their local Exxon dealer. For us, all of those tires came from the same plant in Eastern TX. I'm not sure if the later Atlas radials were as good in this respect as radials were just being promoted by the middle 1970s.

The Exxon station where we bought gas sold a lot of those tires, back then. One time, he showed me some tires he took off. When inflated on the wheel, they looked to be mostly bald, with less than 2/32" tread depth. But when sitting in the "old tires pile", they measured more than 3/32" tread depth. A brand sold by a 2nd-line catalog department store, fwiw.

SO . . . initial quality an be determined by the performance standards cast into the tire sidewall, as to wear, heat resistance, or braking performance. Past that, a good tire can be made better by exceptional attention to maintenance (tire pressures and vehicle alignment), which can also be affected by the roads driven on. "Value" can relate to price, longevity of use, and general reliability while in use. Some "expensive" tires can be a good value, as some tires at a lower price point can also be a good value, depending upon how it is used.

Used tp be that tires with a good balance of wear, ride performance, and durability were what came on new cars, back then. Had to be careful, though, as some of those were more cost-effective to the OEM than not. As some of the 2 ply/4 ply-rated rayon-cord tires were not as durable as a 4 ply-rated nylon-cord tire, but the rayon tires rode smoother and wore less-well, by observation. The later-1960s polyester cords hit the best balance of good ride, no cold flat-spotting, and long wear (due to cord strength) back then. Rayon faded away as "not good enough" and nylon went into more specialized tires, but also evolved out as new tire models were introduced.

Enjoy!
CBODY67