The term 'PART OF CAR'S TRANSMISSION' can be answered with the 7-letter word 'GEARBOX'. A gearbox is a mechanical device that transmits power from the engine of a car to its wheels. It is composed of several gears, shafts, and bearings that work together to transfer power from the engine to the wheels. The gearbox is an essential part of a car's transmission system, as it helps to control the speed and torque of the vehicle.