An axle ratio is obtained by dividing the number of teeth on the drive pinion gear into the number of teeth on the ring gear. For instance, on a 4.11:1 ratio, the driveshaft will turn 4.11 times for every turn of the rear wheels.
The most accurate way to determine the axle ratio is to drain the differential, remove the cover and count the number of teeth on the ring and the pinion.
An easier method is raise and support the rear of the vehicle on jackstands. Make a chalk mark on the rear wheel and the driveshaft. Block the front wheels and put the transmission in Neutral. Turn the rear wheel 1 complete revolution and count the number of turns made by the driveshaft. The number of driveshaft rotations is the axle ratio. More accuracy can be obtained by going more than 1 tire revolution and dividing the result by the number of tire rotations.
The axle ratio is also identified by the axle serial number prefix on the axle; the axle ratios are listed in the dealer's parts books according to the prefix number.