figuring rear axle ratios

[bbatka]

I figure I should re-post this as its own thread, just trying to help somebody who might find this info useful ...
axle ratio for open diffs
OK, the easiest way I have figured out to find the axle ratio.... and its quite accurate. Other people will tell you to do it differently, but I figured this out so I only had to jack up one wheel so you can do this in your driveway....

OK, start by CHOCKING THE FRONT TIRE!!!! Jack one rear wheel. Mark bottom/or top of wheel. Mark the driveshaft and axle w/paint marker. Put the car in neutral...Have a friend spin the tire 10 revolutions. You count the driveshaft revolutions. then move the decimal place to the left 1 space. My driveshaft turned 27 and almost 1/2 tims around, so thats a 2.73 gear ratio. If your shaft turns 35 and a half, figure 3.55 and so on....

If you jack one wheel, and it doesnt turn, its posi....or you forgot to put it in neutral:clonk:

I find that this way gets enough revolutions out of the drive shaft to really get an accurate ratio- 2.56 vs 2.73 and another close pair 2.92 vs 3.08 gears. Hope this helps .....:waving:

 

[bob k. mando]

with an open rear end, i believe you need to multiply by 2 before you shift the decimal as half of the tire rotation is being divided across the carrier to the stationary tire instead of the driveshaft.

for a Posi or Limited Slip:
1. chock the front tires, forward and back
2. put transmission in Neutral
3. jack up both rear tires
4. put jack stands under the frame rails just forward of the rear tires
5. use a piece of chalk to mark the sidewall of the tire and the driveshaft
6. rotate the rear tire by hand 10 revolutions. verify that the off side tire is also turning at the same speed as the 'driven' tire
7. count the driveshaft revolutions while you're doing step 6
8. divide by 10 or shift the decimal one place to the left.