how to determine gear ratio?

[CB]

someone explained it before but I forgot to write it down
Doh... 

[John_Kunkel]

If the car has Sure-Grip, jack both tires clear of the ground and turn a tire one full revolution while counting the driveshaft turns; the number of driveshaft turns is the gear ratio.

If the car has an open differential, jack only one tire clear of the ground and turn the tire two full revolutions while counting the driveshaft turns.

[MoparYoungGun]

Or, you could drive at, lets say 50mph, and by what rpms are reading (that is if you have a tach) and your tire size you can then calculate your gear ratio.