^^ What he said.... rotate the tire two times and count the approx. rotations of the driveshaft.
I was not told the tire needed to go around twice, Thanks.
This link says the same thing, with a bit more detail:
http://www.how-to-build-hotrods.com/gear-ratio.html
"
If your car has an open rear, jack up only one rear tire and leave the other on the ground. Now put a jackstand under the lifted side. Turn the tire and again count the number of turns of the driveshaft, but this time you must
turn the tire two full rotations.
So:
Posi: Jack both wheels off the ground. Turn one wheel 1 complete turn.
- Count the turns on the drive shaft.
Non-posi: Jack one wheel off the ground. Turn the wheel 2 complete turns.
- Count the turns on the drive shaft.
If it takes about 2 and three-quarters turns, it's around a 2.73 gear ratio. If it's a little more than 3, you have 3.08s. If it takes about 3 and a quarter turns of the driveshaft, you have 3.23 or 3.25 gears. Very close to 3 and three-quarter turns, 3.70 or 3.73 gears. This method won't work very well on a posi that's worn-out and won't spin both tires equally.
This method is very handy when looking through junkyards or swap meets. It's good to be able to figure what gear ratio you have without tearing anything apart.
This is a quick and easy way to tell what gear ratio is in your rearend. You might not be able to tell exactly what gear you have, but you'll be very close.
If you want to be accurate, then count the teeth."