Differential gear change?

Dixon

Dixon

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Good evening C body gurus,
I am canvassing for opinions or actual experiences with different rear axle ratios. I have a 1965 Newport wagon, 383 4bbl, 727 transmission, and (by PO) replaced 489 case (newer version than stock) differential. The current gear ratio is 3.23. I am never interested in doing burn outs or racing the vehicle. Rather, we take her to local shows and occasionally longer trips (500-1000 miles round trip). At 60 mph my engine is turning about 2500 rpm. I am considering changing the differential gears to 2.76 or 2.93. I think I have plenty of power to handle the lower ratio and I know my RPMs will go down (pretty much the point to put less stress on her on the open road) and, if I wanted, I could run nearer to 70 mph without such high revs.
One concern I have is I have functioning factory A/C and my current radiator temp hovers at right about 190deg., even in the Florida summer, even with A/C running. Do any of you have experience that would support concern for potential over heating with the lower gear ratio and subsequent lower engine RPMs affecting water pump efficiency and air flow efficiency due to the fan not running as fast? It would be counter production to save some engine RPMs if it caused over heating concerns.
Thanks, in advance, for any input.
 
FWIW I was told by mechanics years ago that after 30-40 mph, the fans don't do anything. Not sure if your concern is in-town or highway driving. I would doubt that changing gear ratio to 2.73 or 2.93 for in town driving would make a huge difference. You indicated long drives. I think you would be fine however others might have experience with this issue. Twosticks suggestion might be less expensive and can be reversed easily.
 
I have 2.76 in several of my cars and will change the others when I get to them. They are the next best thing to having an overdrive. Any highway or interstate trip will make you happy that you changed.
 
Given the weight of your wagon I would not change to a 2.76 rear gear. If you travel to hilly areas your transmission will not be happy with a 2.76 gear. Just my two cents.
 
Many years ago I ran both 3.23 and 3.91 gear in the fury with all else the same. The cooling wasn't effected.
 
Last edited by a moderator:
Your operating temps are NOT anywhere near over-heating. Until they consistently get to 240*F, don't worry about it. In our '66 Newport Town Sedan, I drove that things all over western TX at 70-90mph in the summer with no issues at all. I also have a '70 Monaco Brougham, same things.

The Newport is a base 383 2bbl with the stock 14" tires and 2.76 rear axle ratio. I calculated it to be 28mph/1000rpm. The Monaco with a 383 4bbl, P225/75R-15 tires and 3.23 rear axle ratio to be 25mph/1000rpm. The Monaco has the factory dual exhaust, but at crusing speeds, that should not be an issue.

Your wagon is not that much heavier than a 4dr sedan, BUT the difference is in the wind resistance of the wagon body and the big hole it leaves in the wind, from the more square rear body shape.

Considering that a pumpkin swap on an 8.75" Chrysler rear end might cost approx $1000.00 (parts and labor), in round numbers, how long would it take for a potential 5% increase in fuel economy take to pay for itself? Lowering the cruise rpm will marginally affect engine longevity/durability as any rpm will be at or near the rated torque peak.

Personally, I would focus on better fuel atomization and spark efficiency. For example, an Edelbrock AVS2 has much-better fuel atomization than any 1965 Carter carburetor did. Then some NGK V-Power spark plugs do a better job of getting that mixture fired-off with more spark exposed to the mixture. A better POW. Only thing better would be the NGK Iridium fine-wire spark plugs (for a bit more money). I put the V-Power plugs in all of my cars when I discovered them in the 1980s. Might convert to some kind of electronic ignition, too. End result should be a more efficient conversion of fuel to power.

When was the last time a timing chain was put into the motor?

It used to be that 1960s factory a/c systems (GM and Chrysler) usually took about 2mpg to run. Replacing the existing RV-2 compressor with a newer Sanden unit can decrease that a bit, I suspect. Less power consumption by the compressor.

I concur on the 15" radial tires addition in P225/75R-15 size. They'll fit fine on 15x6 wheels. Just need to find some factory wheel covers you like for them.

Just some thoughts and observations,
CBODY67
 
I like 3.23, 2.94 is also good.

2.76 is too far the wrong way, uses more fuel and kills the torque. Don't do it!
 
Dixon said:
Good evening C body gurus,
I am canvassing for opinions or actual experiences with different rear axle ratios. I have a 1965 Newport wagon, 383 4bbl, 727 transmission, and (by PO) replaced 489 case (newer version than stock) differential. The current gear ratio is 3.23. I am never interested in doing burn outs or racing the vehicle. Rather, we take her to local shows and occasionally longer trips (500-1000 miles round trip). At 60 mph my engine is turning about 2500 rpm. I am considering changing the differential gears to 2.76 or 2.93. I think I have plenty of power to handle the lower ratio and I know my RPMs will go down (pretty much the point to put less stress on her on the open road) and, if I wanted, I could run nearer to 70 mph without such high revs.
One concern I have is I have functioning factory A/C and my current radiator temp hovers at right about 190deg., even in the Florida summer, even with A/C running. Do any of you have experience that would support concern for potential over heating with the lower gear ratio and subsequent lower engine RPMs affecting water pump efficiency and air flow efficiency due to the fan not running as fast? It would be counter production to save some engine RPMs if it caused over heating concerns.
Thanks, in advance, for any input.
Click to expand...
Thanks all for the thoughtful and insightful sharing and advice. The more I think about it, the more I'm leaning toward the "it ain't broke so why try to fix it?" mentality. I don't run her on Interstates anyway and, if I did, I can drive 65 and there is another lane or two the faster vehicles can use to pass me. Happy Thanksgiving all. Stay safe.
 
I have a 3.23 gear with my '67 383 and P235/75/R15 tires. They are a good combination.

Wait until next tire change and then upgrade to the taller size. Easy peasy.
IMG_2181.JPG
 
Dixon said:
Thanks all for the thoughtful and insightful sharing and advice. The more I think about it, the more I'm leaning toward the "it ain't broke so why try to fix it?" mentality. I don't run her on Interstates anyway and, if I did, I can drive 65 and there is another lane or two the faster vehicles can use to pass me. Happy Thanksgiving all. Stay safe.
Click to expand...
Did the car come with 15's? I'd verify your speedo. Lots of GPS apps for phones out there.

I think 15s were an option but 14s were the norm by and large, especially on a Plymouth or Dodge. If someone swapped it to 15s and didn't change the speedo gear in the transmission, you could be indicating 60, but doing closer to 70 +/-.

You're sure you have a 3.23? Your RPM/MPH is dead on for 235 75 15 tires with a 3.55 gear.

Kevin
 
This topic is bound to gather a litany of responses. It all boils down to your personal choice as far as which suggestion appeals to you. Change tire/wheel size or the gear ratio from the original, and you need to check speedo drive gear size (Chosen by teeth and displayed by colour, yellow, green, etc. There are charts to follow.). This is a simple correction and change. Changing gear ratio requires more work, and will definitely reduce your rpm. As far as the reference to "climbing", I have lost track of the number of mountain passes I have traversed pulling a camper with various B and C bodies, with big block motors and 2:76 gears. No problem and no overheating. I have also done it with 3:23's. Both worked fine. It is simple, if you have a heat concern slow down a bit and carry on! It only takes a few miles per hour to accomplish this, and every car has its sweet spot. Regarding 2:72 vs 3:23, there can be no doubt the the latter is more peppy and requires more rpm. The former is "slower" by direct comparison but tends to lower rpm and have better fuel mileage. Let us not forget that you are driving a heavy car, on a public street, will probably not be drag racing (although we all get a bit spirited now and then), and have a monster toque motor under the hood. Use you goals as a decision reference, your wallet as a decision factor, and go have fun.
 
twostick said:
Did the car come with 15's? I'd verify your speedo. Lots of GPS apps for phones out there.

I think 15s were an option but 14s were the norm by and large, especially on a Plymouth or Dodge. If someone swapped it to 15s and didn't change the speedo gear in the transmission, you could be indicating 60, but doing closer to 70 +/-.

You're sure you have a 3.23? Your RPM/MPH is dead on for 235 75 15 tires with a 3.55 gear.

Kevin
Click to expand...
I doubt it on the 15" wheels. They were on it when I bought it.

My speedo doesn't work properly so I am using an app. And who knows how accurate the tach is?

I have never looked at the actual gear stamp. I'm just going by the tag on the differential. No one knows for sure what any PO's have done. Considering some of my other discoveries anything is possible. Lots of unknown variables.

Thanks,
 
I have an 87 5th Ave 318 with stock 2.20 rear gear. It runs cool.
It "was" very nice at 80 mph, but a real dog in town. As in the most aggravating car I've ever driven. Worse than an old 36 hp VW beetle. Really, I'm serious.

So, I moved the shift points up 15 mph, just fine in town now.
Leaves the stop light at part throttle just fine, stays in first and second longer at part throttle. The 1/2 shift is at 30 instead of 15, the 2/3 shift is at 45 instead of 30. Accelerates through town is just fine now. Part throttle acceleration at 70 mph is fine for 10 hours at a time. Cruise control doesn't gear hunt, never did. Nice and quiet, the way I like it.

But, it also has a lower first gear than the 2.91 cars had a few years earlier.

Love the car now. 24 mpg at 65 mph. 22 at 75. 20 at 85. Went from 15 in town to 17 in town.
 
69 Bonneville wagon, th400 428 4v. 2.56 posi was so nice for family road trips. Even set the cruise control at 90mph with the AC on. Whew
 
Craven said:
69 Bonneville wagon, th400 428 4v. 2.56 posi was so nice for family road trips. Even set the cruise control at 90mph with the AC on. Whew
Click to expand...

69 Pontiac 428 you say?
Well, I guess it was nice with the 2.56.
In 75 I drove a 69 Bonneville 2dr th400 428 with 3.23 gear, quite the hotrod it was. Probably with any rear gear.
I wish Chrysler had offered that 2.56.
 
If I remember correctly, 1976 Dodge Royal Monaco Cop car was available with 9.25" 2.92:1 sure grip and a 400. I had one after it was retired. One of my favourite cars,...bit slow off the line but what a solid and fairly agile machine. Seems strange to use the word "agile" with something that big,...big still one of my favourites. Still in college and drove the crap out of it. Sold it when it cost me thirty bucks to fill the tank,...Ha!
 
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