Rear Gear Calculations (for Auto)

[vms4evr]

Ok, seeing as how this comes up a bit with "What is the speed/rpm with a given gear?" Here's the answers I got when doing the math for my gear change.

Assumption: Tire Diameter = 25.7in (275x40/17), Drive Ratio is 1.0, Overdrive Ratio is 0.7 (this is for an auto of course). 5400rpm is max with factory PROM/Chip.

In Drive (ratio is 1.0)

Ratio 2.59 3.54 3.75

MPH RPM RPM RPM
----- ------ ------ ------
45 1524 2083 2206
55 1862 2545 2696
65 2201 3008 3187
75 2540 3471 3677
85 2878 3934 4167
95 3217 4397 4658
105 3555 4860 5148
115 3894 5322
125 4233
135 4571
145 4910
155 5249
165

In Overdrive (Ratio is 0.7)

Ratio 2.59 3.54 3.75

MPH RPM RPM RPM
----- ------ ------ ------
45 1067 1458 1544
55 1304 1782 1888
65 1541 2106 2231
75 1778 2430 2574
85 2015 2754 2917
95 2252 3078 3260
105 2489 3402 3604
115 2726 3726 3947
125 2963 4050 4290
135 3200 4374 4633
145 3437 4698 4976
155 3674 5022 5319
165 3911 5346


The formula works as follows:

For Drive (ratio 1.0)
RPM = ((RE Ratio * MPH) / Tire Diameter) * 336

For Overdrive (ratio 0.7)
RPM = (((RE Ratio * MPH) / Tire Diameter) * 336) * OD Ratio

So if you want to calculate different rear gears or final drive ratios just plug it into the formula. Or use the tools on this website or many others.

Graham

 

[Ken]

Thanks for doing the homework Graham.

_ken

 

[Corvette-Pilot]

Very Helpful Info ! :BOW

I am also considering a rear end change and this will help a lot.

Question:
What kind of rear end is in my car?
1994 coupe, auto, 2.59 gears

What ratios can I put into this rear end?

Thanks guys

 

[vms4evr]

Pilot,
The default for an auto trans is the 2.59. If your RPO codes show a G92 then you have the factory Performance Axle which is 3.07.
You only have 2 other options. 3.54 or 3.75 (aftermarket).
The MPH/RPM numbers posted above are for my92. Starting in 93 they went with the staggered tire sizes and you should have 285x40/17 on the rear. So the numbers will be off slightly. You're rear tires are about a half inch taller than mine. I think 26.3in, but don't quote me. Just check any tire vendor for the height.
Graham

 

[Corvette-Pilot]

Thanks !

I will have to look up the options again to be sure of what I have.
I will also do the old rotate and count trick to be sure no one already changed the gears.