Respectfully, some of your information (#4 above) is a modern take on prior specs, which can make it a bit dated and inaccurate, from what I've seen since 1968.
Yes, tires used to be rated in inches of section width. The 8.25x14 tire was supposed to be 8.25" wide, although no government standards existed at that time.
The alpha-numeric system started with the 1968 model year. A letter for total weight capacity, numbers for the aspect ratio and rim diameter. From an article in CAR LIFE magazine about this, it was stated that the letters would make it easier for replacement tire purchasers to make sure they got a tire with enough weight capacity for their vehicle. Also, "Minimum Tire Size" decals started to appear on the lower B-pillar area of the lh front door, plus pressure recommendations, too. NO need to look at an owners manual for these things. The orientation being that the minimum tire size at the stated (minimum) tire inflations would be enough to carry the weight of the car, passengers, and trunk loadings. In some cases, OEMs had equipped vehicles with tires a notch too small in prior times, relying on the dealers to order the better size (which many obvioisly did not). So this was about tire safety, in many cases.
Radial tires, before 1968 and afterward, were listed in metric (millimeter) sizes for the section width, but later included aspect ratio, too. Hence, normal radial would be "215x14" or "215/70x14". NO P-metric designations yet!
So, you can convert the old "inches" width to Metric width by . . . Inches x 25.4 = metric width. Then as now.
With the then-coming increased emphasis on EPA fuel economy ratings, it was determined that tires needed to have higher inflation pressures (high than 32psi for "standard load" tires). To indicate that change, the "P-Metric" sizing was introduced. With its additional nomenclatures and such, too.
P = Passenger ___ = Section width __ = Aspect Ratio _ = Construction Type - __ = Rim Diameter With a "Load Ratio" followed by a "Letter" designating Speed Rating.
P225/75R-15 for a radial P225/75D-15 for a "diagonal ply" (bias ply) 89T
Speed Ratig started with "S" = 112mph. "H" was about 135mph "V" to "Z" beihg higher.
There is a chart somewhere decoding the metric weight rating. P-Metric tires have a normal inflation pressue max of 35psi for a "Standard Load" tire, with about 40psi being an "Extra Load" passenger tire, similar to a 6-Ply rating or sorts, designated by "XL". There possibly is a more detailed breakdown of these things at places like
www.tirerack.com.
As to "Revs/mile", my old BFG Radial T/A pamphlet lists it as "Rev/mile at 45mph". IF you get into the old SAE Specs for vehicles, it lists the "Revs/mile" rating for the tires on that particular vehicle. With the older bias-ply tires, the tires would expand as the speed increased, which radials minimized due to their radial construction. Bias-belteds would be similar.
Otherwise, you can look at the diameter of the tires, but the diameter can differ between "base diameter" or "compressd diamter" with the tire on the car and the car on the ground. An old "Hot Rod Annual" had a method to determine "rolling diameter" by marking the tire's tread and then rolling the tire one revolution, then measuring the distance between the marks to determine the tire's rolling circumference. Which could then be use to determine what we now call "Rev/mile". Needed one of two things to do all of this. A clean sheet of paper or a slide rule.
THEN, using these figures, you could do other computations to determine MPH per 1000 rpms, to determining speeds in gears and shift points. No tach needed for these things, after figuring all of these things. THEN you could do "time/distance" checks for speedometer speed calibration checks. In later times, I would use the mile markers on the Interstates to do this. 10 miles at a constant speed of 60mph works nicely as 66ft/second = 60mph = 60 seconds/mile. 10 miles usually worked best, when I could find a section of highway that long, which was flat, but 5 miles can work well too. ALL in a time well before GPS!
Somewhere, I made a list of tire sizes and revs/mile from the many AMA Specs documents I have found online, many from
www.hamtramck-historical.com. But what i had done earlier usually pointed to 8.55x14 and P225/70R-15 being the same basic revs/mile, as in 750-760revs/mile,. basically. FWIW, those "AMA Specs" documents are kind of like a condensed FSM's specs, all in one place.
OTHERWISE . . . "Revs/Mile" specs are listed, generally, for each tire size in the "Specs" section for the particular tire at
www.tirerack.com. In most cases, both the revs/mile and "Diameter", too. But to me, depending on the sidewall deflection on radial tires, the "Revs/Mile" is more accurate for our purposes, to me.
One ALL of these things are known, with respect to speedometer gears in the transmission tailshaft, with changing gears and/or tire sizes, you can THEN see what gear combination might be needed to keep the speedometer cable revs/mile (and speedometer calibration) in specs.
Rear axle ratio and tire revs/mile means "Driveshaft revs/mile". Using the starting point in the FSM for speedometer gears/tire size/axle ratio, then use percentages to see what speedometer gears are needed (drive and driven). "Driven" being the gear the cable end attaches to at the transmission.
There are several online interactive charts to indicate tire dimensions and revs/mile just by inputting the information, then getting a comparison (visual and number) of the starting and ending tire size. The first such chart I found was at
www.miata.net/garage/tirecalc.html. Its firgures can be "theroretical" rather than actual, though, so using the spec charts at TireRack can be a bit more accurate, by observation.
Starting with the alpha-numeric designations of 1968, government standards for tire sizing and such were put in place. With a max variation of 7% from the stated spec being allowed.
Respectfully, almost all of the information you desire IS online, just have to know where to look for it and compile it, by observation.
Other than the Nexens and Hankook whitewalls, there can be more white letter tires in our sizes, in some brands. By observation, Cooper Trendsetter SE USED to have 14" whitewall sizes, with the OEM narrow treads the cars came with. But I believe they had been discontinued over time.
Tire tread width is supposed to be within plus or minus 1" of the rim width. Rim width being measured BETWEEN the beads rather than on their outer edges. A 6" rim width might be better than a 5.5" rim width on a 8.55x14 tire, but NOT necessary. As for handling, probably would not be able to feel any real difference between a 5.5" and 6.0" rim width, I highly suspect. Although with a bit more arc in the narrower rim, might get a slightly better ride.
Now, in that subject . . . when I put the 15x7 Magnum GT wheels on my '80 Newport, I opted to keep the P215/75R-15 standard size tires. With the wider rim, it made the tires' sidewalls pretty much vertical, with little radial bulge. Which seemed to increase steering response more than I expected. A neat way to get better performance from the inexpensive tires I put on them.
Sorry for the length, lots of dynamics to mention in this area.
Respectfully,
CBODY67
