A transmission where the gears are shifted automatically by the vehicle and controlled by a torque converter. The torque converter replaces the clutch. It connects and disconnects the engine from the drive wheels while selecting the proper gears for the speed of the car. These transmssions come in 3, 4 and 5 speeds. The more gears involved, the smoother the acceleration. With 4 and 5 speed transmissions, the highest gear is for overdrive. Overdrive reduces the revolutions per minute (RPM) and provides better gas mileage while on the highway.