Differential side gear

A differential is a gear assembly that splits engine torque to the driven wheels while letting them turn at different speeds.

When the vehicle corners the outer wheel travels farther than the inner one; the differential gears allow that speed difference while still driving both wheels, and the final drive gears also reduce speed and multiply torque.

The differential sits in the drive axle of a truck or bus. The correct parts match the specific axle model and gear ratio.