An axle’s ratio describes the number of turns required by the driveline to turn the wheels once. Technically, ratios should be written as 3.90:1 , which means the driveline turns 3.9 times for every one turn of the wheels.
So, the lower the axle ratio, the faster the truck will move.
An axle’s ratio is determined by the number of teeth on the ring gear and the pinion gear. If you divide the ring gear tooth count by the pinion gear tooth count, you get the ratio of that gear set.