Lesson 2/Learning Event 2
The size of the gears used in final drives depends on the needs of the vehicle. The
relationship of the pinion and ring gear is known as the gear ratio. The ratio of gears is
obtained by dividing the number of teeth on the pinion into the number of teeth on the
ring gear. This will show how many revolutions the pinion makes when the ring gear
makes one revolution. For example, if the pinion has 10 teeth and the ring gear has 50
teeth, the ring gear has five times as many teeth and the ratio is 5:1 (5 to 1).
The ratio of automobile drive axles ranges from 2.5:1 to 5:1. Large trucks have gear
ratios ranging from 5:1 to 15:1.
Lighter vehicles that are not required to haul heavy loads can use a comparatively low-
ratio set of gears. For example, a small commercial truck may have a gear set with a
ratio of 5:1. This ratio can easily be obtained with one set of reduction gears.
Larger trucks need gear ratios up to as much as 15:1. This would mean that if 10 teeth
were on the pinion, there would have to be 150 teeth on the ring gear. A ring gear of
that size would take up too much room. Instead, the ratio or total reduction is divided
up between two sets of reduction gears. This type gear arrangement is known as the
double-reduction drive. This way, if the bevel gears supply about a 3:1 reduction and the
second reduction gearing a 5:1 reduction, the overall ratio would therefore be 15:1 and
none of the gears would take up too much room.
The double-reduction axle should not be confused with the two-speed axle. Some
commercial trucks use a drive axle that has two different ratios or reductions. The driver
can select the ratio needed with controls that are located in the truck cab. This type axle
has a "low" range and "high" range. The ranges are actually two different ratio
reductions. Two-speed axles are known as dual ratio and double reduction-dual ratio
On live axles, one wheel must turn at a different speed than the other as the vehicle goes
around a corner. Additional gearing is required to allow for the difference in the speed
of the wheels. This gearing must also continue to drive both wheels at the same time.
The gearing assembly designed to do this job is called a differential. The differential
assembly is mounted in the axle housing and is bolted to, and driven by, the final drive