(written by Slanted_Mind)

If you are lucky, the factory tag denoting you gear ratio may be hanging from one of the differential cover bolts.  In the absence of this tag, or any other options tag that may or may not be under the hood or in the door jamb of your vehicle, you can use the following procedure to check your gear ratio:

First, determine whether you have a Sure-Grip/Locking differential (Commonly reffered to as "Positraction" by the Chevy realm), or an Open differential ("Peg leg" or "One wheel peeler").

Determining differential type

1) Block the front tires.
2) Place the transmission in Park (auto) or in gear (manual).
3) Jack up the rear axle (both tires off the ground).
4) Try to turn one tire by hand.  If it is very difficult to turn, or will not turn at all, you have a Sure-Grip or locking differential.  If you can easily turn the wheel, and the wheel on the opposite side turns the opposite direction, you have an open differential.

Now that you have determined your axle type, on to determining the gear ratio.

For an Open Differential

1) Block the front tires.
2) Make an easily identifiable mark on both the tire and the driveshaft (chalk, Majic Marker, whatever you have) so you can count their respective revolutions when they are turned.
3) Place the transmission in neutral.
4) Jack up one rear tire (the one you marked in step 2).
5) Spin the tire two full revolutions, counting the number of revolutions the driveshaft makes.
6) The number of revolutions the driveshaft makes is your gear ratio.  (E.G. 3-1/4 turns = 3.23:1, 3-1/2 turns = 3.55:1, etc.)


For a Sure-Grip/Locking Differential

1) Block the front tires.
2) Make an easily identifiable mark on both the tire and the driveshaft (chalk, Majic Marker, whatever you have) so you can count their respective revolutions when they are turned.
3) Place the transmission in neutral.
4) Jack up both rear tires.
5) Spin the tires (both should turn the same direction when one is turned) one full revolution, counting the number of revolutions the driveshaft makes.
6) The number of revolutions the driveshaft makes is your gear ratio.  (E.G. 3-1/4 turns = 3.23:1, 3-1/2 turns = 3.55:1, etc.)