way to estimate rear diff ratio?

[evan]

Is there a way to figure out pretty accurately what gear ratio a rear end has based on counting tire and driveshaft revolutions? I just counted on my truck and in 10 revolutions of the tire the driveshaft made 17 revolutions.

[dropit69]

they say rotate it once and multiply the number of times the drive shaft rotates times 2...so you say your drive shaft rotated 1.7 x 2 = 3.40 thats just a rough guess way..thats what i found out googling it..lol

[evan]

Cool. So I probably have 3.43's.

[chicane67]

Turn both rear tire at the same time (if not a posi it might take two people)...

Count the revolutions of the drive shaft per one turn of the tire.

If it goes 3 and 3/4 turns... its 3.73's. If it goes a little over 4 turns... its 4.10. You get the idea...

[andrewb70]

Or just take the cover off. The ratio is stamped on the ring gear. You will see something like 41 10. 41 teeth on the ring gear, 10 on the pinion, hence 4.10s.

Andrew

[Adam's 55 Chevy]

tire height x rpm x 0.002975 / speed = rear end ratio.

[70camaro406]

Turn both rear tire at the same time (if not a posi it might take two people)...

Count the revolutions of the drive shaft per one turn of the tire.

If it goes 3 and 3/4 turns... its 3.73's. If it goes a little over 4 turns... its 4.10. You get the idea...

That's the easiest way I've found. Been doing it that way for long time. Clean and quick, it only takes a couple of minutes. Might need a marker or chalk to mark on the tire and the driveshaft. I always mark the tire at the bottom, mark the driveshaft at a spot you can see easily, roll the car until the tire mark makes one revolution while counting the driveshaft revolutions. One tire revolution, about 2 3/4 rotations = 2.73 (my first rear). I installed another rear with 4.10's, tried it again for proof that it works, and driveshaft spun just over 4 times = 4.10.

[70camaro406]

tire height x rpm x 0.002975 / speed = rear end ratio.

That might work with a stick, but not so well with a high stall converter due to slippage variances.

[johnsma22]

Wallace Racing (http://www.wallaceracing.com/Calculators.htm) has more calculators for stuff like this than you could throw a stick at. Check it out.