The transmission has different pairs of gears in a housing. These gears allow a car to drive at different speeds. The tramsission oil lubricates and cools the gears. A low gear moves a car from stop. Other higher gears heep the engine speed to proper levels while accelerating. A car can go at a less maximum speed in a lower gear, but it has greater accelerating ability and hill-climbing ability. To save fuel for highway driving,  most cars have overdrive gears which decrease enginge speeds to minimum practical levels. Overdrives should be used for constant speed driving on level roads but should not be used for trailer-towing, hill-climbing and accelerating to pass another car.

Manual transmission has a clutch which is controlled by the clutch pedal. When a driver depresses the clutch pedal by foot, the clutch disengages the engine from the transmission, and the driver can shift the gears, start or stop the car. When the foot is off the clutch pedal, the engine transmits power through the clutch to transmission to the drive wheels.

Automatic transmission selects the proper gear by itself. Instead of using a clutch, the automatic tramsimission uses a torque converter or fluid coupling to disconnect the engine and transmission during shifting and starting. Automatic transmission fluid is used to operate and lubricate the transmission gears.

When a car turns a corner, the differential allows one drive wheel to turn faster than another but splits the driving force between both wheels. The differential is housed in the transaxle with the tramsmission in front wheel drive cars. In rear wheel drive cars, the differential is part of the rear axle assembly.