The purpose of the final drive gear assembly is to provide the final stage of gear reduction to decrease RPM and increase rotational torque. Typical final drive ratios can be between 3:1 and 4.5:1. It is because of this that the wheels never spin as fast as the engine (in almost all applications) even when the transmission is in an overdrive gear. The final drive assembly is connected to the differential.
In FWD (front-wheel drive) applications, the final drive and differential assembly are located inside the transmission/transaxle case. In a typical RWD (rear-wheel drive) application with the engine and transmission mounted in the front, the final drive and differential assembly sit in the rear of the vehicle and receive rotational torque from the transmission through a drive shaft. In RWD applications the final drive assembly receives input at a 90° angle to the drive wheels. The final drive assembly must account for this to drive the rear wheels. The purpose of the differential is to allow one input to drive 2 wheels as well as allow those driven wheels to rotate at different speeds as a vehicle goes around a corner.
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