auto

An auto transmission is a gearbox that selects and engages gear ratios without manual input from the driver. Instead of a clutch pedal and gear lever, the driver controls only throttle and brake; hydraulic pressure, mechanical governors, or electronic control modules detect vehicle speed and engine load, then automatically upshift or downshift within a fixed set of ratios. This contrasts with a manual transmission, where the operator must deliberately engage each gear.

Most automotive autos use a planetary gearset with multiple friction clutches and bands to achieve different ratios from the same physical arrangement. A fluid coupling or torque converter replaces the mechanical clutch, using pressurized transmission fluid to transmit engine torque. The number of forward speeds varies: three-speed and four-speed autos were common through the 1990s; modern vehicles often have six, eight, or even ten speeds to improve fuel economy and performance across wider engine speed ranges.

Electronic autos, now nearly universal in passenger cars, use solenoid-controlled valves to direct fluid flow and apply the correct clutches and bands. Older mechanical governors relied on centrifugal force to sense road speed and engine load; these units shift more abruptly and offer less control. Modern transmissions communicate constantly with the engine control unit, adjusting shift points based on throttle input, coolant temperature, and driving mode selections.

Common failures in autos include fluid degradation, which reduces hydraulic pressure and causes slipping or delayed shifts; seal leaks that reduce fluid level; and solenoid or valve body stiction. Overheating accelerates fluid breakdown; sustained towing or heavy traffic can push transmission fluid above 200°F (93°C), shortening service life. Servicing intervals vary: some modern autos claim 100,000-mile drain intervals, while others are sealed units with no scheduled maintenance.

The term auto is clipping, shorthand for automatic, common in trade speech and casual use. In formal technical writing, automatic transmission remains preferred. The broader category includes continuously variable transmissions (CVTs), which have no fixed gears, though these are sometimes distinguished separately in specification sheets.