rear gears

will34

Member
Jun 16, 2019
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I have 1982 302 engine with a 1969 c4 tranny. The rear is 8 inch 300 gears. The rpms at 50 mph is 2200. My question is what would be the difference in rpm's if I change the gears to 280's. The motor has small cam. I believe the # on the cam 303.

Thanks
 
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7991LXnSHO

wanna catch the space herp
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Sep 1, 2010
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The transmission’s top gear is 1:1 ignoring slippage. Measure your tire height, then google for a rear end RPM calculator. The math should be pretty clear so that someone should have put it in a web page for us as a calculator.
The math just makes my head hurt today, :)
 

rbohm

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Apr 12, 2002
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the rpm difference is very slight, something like under 50 rpms if i recall correctly.
 

will34

Member
Jun 16, 2019
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ontario
Thank you for the replies. There is about 125 rpm difference at 50 mph. Now I can decide if the lower rpm is worth the cost or new gears.
Thanks
 

rbohm

SN Certified Technician
Apr 12, 2002
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for 125 rpm? not worth the effort or the money, since you will never notice the difference.
 

65ShelbyClone

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Sep 9, 2000
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I have 1982 302 engine with a 1969 c4 tranny. The rear is 8 inch 300 gears. The rpms at 50 mph is 2200. My question is what would be the difference in rpm's if I change the gears to 280's.

All you have to do is 2.80 ÷ 3.00 to get a percentage difference and then multiply that by your 2200rpm.
 

KRUISR

5 Year Member
Apr 16, 2015
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AOD with 3.73s will run at 1600 RPM at 50 mph. With this combo 2200 RPM is about 70 mph.

AOD and your 3.00s will run 1300 RPM at 50 mph.
 

KRUISR

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Apr 16, 2015
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Will34 has a C4 trans.

Yes I know.

He was looking for gear options to rev lower. My point was that with an AOD he could go up in gears and have more grunt at lower speeds (higher gear ratio) and still get lower RPMs while cruising. To get lower cruising RPMs with lower than a 2.80 will make the car a real dog out of the hole (unless he has a torque monster for an engine).
 
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DBegley

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Jun 24, 2017
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What is it you are wanting to accomplish? To get lower RPM at highway speeds? To save gas consumption? You should have my setup. A 3.89 rear end that is doing more like 3200 RPM at 60. Gas Mileage? Forget that question. I agree with the statement that what you would save in RPM is not going to justify the cost and hassel of swapping out the rear end. Bet you wouldn't even notice it. I am pondering swapping out my 4 speed for a 5 or 6 speed to get a highway gear. I hate to lose my Toploader though. I'd be looking for a newer AOD transmission. That makes more sense to me. A 2.80 rear gear? Most of us owners want a lower gear for acceleration.