The function of the transmission in a vehicle is to transfer power from the vehicle's engine to the driveshaft and rear wheels. The speed and torque of the wheels changes in relation to the speed and torque of the engine due to the gears in the transmission.
Transmissions are either automatic or manual. In a vehicle with a manual transmission, the driver shifts from one gear to the next to control the speed and torque of the engine and wheels. The driver uses the shift lever and clutch inside the vehicle to move through the gears. In an automatic transmission, the transmission automatically shifts through different gears based on the use of the gas and brakes from the driver.