I thought I had a 3:31 12 bolt rear. In trying to get the speedometer to read correctly I'm 10 miles per hour slow.
How do I determine what my rear gear is ?
Thanks.

Easiest way is to jack both rear tires off the ground and mark a reference point on one where it meets the ground. Make a similar reference mark on the driveshaft and then rotate a rear tire one full turn while counting the number of revolutions of the driveshaft. Not an exact science but should get you in the neighborhood.

Easiest way is to jack both rear tires off the ground and mark a reference point on one where it meets the ground. Make a similar reference mark on the driveshaft and then rotate a rear tire one full turn while counting the number of revolutions of the driveshaft. Not an exact science but should get you in the neighborhood.

This will work if it's a Posi rear, but not if it's open. If it's a posi rear, you can just jack both wheels off the ground, and when you rotate one wheel, the other will rotate with it. Then you just count the rev's of the driveshaft for one rev of the wheel and that's your axle ratio (3 1/2 turns = 3.55 or 3.42, just over 3 = 3.08, etc.)

For an open rear, if you try to do this, you'll spin one wheel and the other will just spin backwards. (this is the differential working). In this case, you can either jack up just one wheel or have someone hold the other wheel while you spin one. Also, in this case the axle ratio is equal to the number of driveshaft revolutions multiplied by 2. (1.75 turns on the driveshaft = 3.55 or 3.42, 1.5 turns = 3.08). The factor of two comes in because of the differential.

Alternatively, if you REALLY want to know, you can pull the read end cover and either count the teeth or read the stamping on the ring gear, which should tell you the teeth on it.

In this case, you can either jack up just one wheel or have someone hold the other wheel while you spin one. Also, in this case the axle ratio is equal to the number of driveshaft revolutions multiplied by 2. (1.75 turns on the driveshaft = 3.55 or 3.42, 1.5 turns = 3.08). The factor of two comes in because of the differential.

You could also turn the wheel two complete revolutions and read the ratio directly. If you want a more accurate determination, rotate it 20 times.

look for a stamp on the rear of the housing. mine was on the passenger side i believe. it should tell u if its a posi or not and the exact gear ratio. it will be in the form of a number and letter code. just punch in that code in google and u should find it.