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 ratio, the driveshaft will turn 4.11 times for every turn of the rear wheel. 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 pinion.
An easier method is to jack and support the vehicle so that both rear wheels are off the ground. 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 one 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 one tire revolution and dividing the result by the number of tire rotations.
In order to rotate the driveshaft, 4wd vehicles must have their transfer cases set in Neutral or 2wd. On the full-time 4wd, you should raise and support the truck so all wheels are off the ground.
The axle ratio is also identified by the axle serial number prefix on the axle; the axle ratios are listed in dealer's parts books according to prefix number. Some axles have a tag on the cover.