Turning the drivers side hub one turn while watching the pinion yoke to see how many turns that makes will give you a pretty good idea of the ratio. For instance ,one turn on the hub may give about 2 3/4ths turns on the pinion = 2.73 gears etc.
The rearend gear ratio refers to the relationship between the ring gear and the pinion gear. By simply dividing the ring gear tooth count by the pinion gear tooth count, the ratio is determined. For example, if we divide a ring gear with 41 teeth by a pinion gear with 10 teeth we find that the gear ratio is 4.10:1 (41/10 = 4.10).
Read more: Gear Ratio Calculating - Tech Article - Chevy High Performance Magazine