Many rear-wheel-drive and four-wheel-drive vehicles are equipped with automatic transmission. Automatic transaxles, which combine an automatic transmission and final drive assembly in a single unit, are used on front-wheel-drive, all-wheel-drive, and some rear-wheel-drive vehicles.

An automatic transmission or transaxle selects gear ratios according to engine speed, power train load, vehicle speed, and other operating factors. Little effort is needed on the part of the driver, because both upshifts and downshifts occur automatically. A driver-operated clutch is not needed to change gears, and the vehicle can be brought to a stop without shifting to neutral. This is a great convenience, particularly in stop-and-go traffic. The driver can also manually select a lower forward gear, reverse, neutral, or park. Depending on the forward range selected, the transmission can provide engine braking during deceleration.

The most widely used automatic transmissions and transaxles have three forward speeds and neutral, reverse, and park positions. Four-speed units that offer an overdrive fourth gear have become increasingly popular. Overdrive occurs when less than full revolution of the transmissionĄ¯s input shaft produces one full revolution at the output shaft. The purpose of overdrive is to improve fuel economy and reduce engine speed and noise. Most new automatics also feature a lockup torque converter.

Until recently, all automatic transmissions were controlled by hydraulics. However, many new systems now feature computer-controlled operation of the torque converter and transmission. Based on input data supplied by electronic sensors and switches, the computer sets the torque converterĄ¯s operating mode, controls the transmissionĄ¯s shifting sequence, and in some cases regulates transmission oil pressure.