In trucks, it can vary from 5 to 1 to as much as 11 to 1. The gear ratio of a final drive having bevel gears is found by dividing the number of teeth on the drive gear by the number of teeth on the pinion. In a worm gear final drive, you find the gear ratio by dividing the number of teeth on the gear by the number of threads on the worm. Most final drives are of the gear type. They permit the bevel drive pinion to be put below the center of the bevel drive gear, thereby lowering the propeller shaft.
Worm gears allow a large speed reduction and are used extensively in larger trucks.
Spiral bevel gears are similar to hypoid gears. They are used in both passenger cars and trucks to replace spur gears that are considered too noisy. We will briefly review some of the high points of that chapter here and describe some of the more common types of gear differentials applied in automobiles and trucks. The purpose of the differential is easy to understand when you compare a vehicle to a company of sailors marching in mass formation. When the company makes a turn, the sailors in the inside file must take short steps, almost marking time, while those in the outside file must take long steps and walk a greater distance to make the turn.