Most cars sold in the United States since the 1950s have been equipped with an automatic transmission. This has, however, not been the case in Europe and much of the rest of the world. Automatic transmissions, particularly earlier ones, reduce fuel efficiency and power. Where fuel is expensive and, thus, engines generally smaller, these penalties are more burdensome. In recent years, automatic transmissions have significantly improved in their ability to support high fuel efficiency but manual transmissions are still generally more efficient.

      Most automatic transmissions have a set selection of possible gear ranges, often with a parking pawl feature that will lock the output shaft of the transmission.

      However, some simple machines with limited speed ranges and/or fixed engine speeds only use a torque converter to provide a variable gearing of the engine to the wheels. Typical examples include forklift trucks and some modern lawn mowers.

      The predominant form of automatic transmission is hydraulically operated, using a fluid coupling or torque converter and a set of planetary gearsets to provide a range of torque multiplication.

HISTORY AND IMPROVEMENTS
     Oldsmobile's 1940 models featured Hydra-Matic drive, the first mass-production fully automatic transmissions. Initially an Olds exclusive, Hydra-Matic had a fluid coupling (not a torque converter) and three planetary gearsets providing four speeds plus reverse. Hydra-Matic was subsequently adopted by Cadillac and Pontiac, and was sold to various other automakers, including Bentley, Hudson, Kaiser, Nash, and Rolls-Royce. From 1950 to 1954 Lincoln cars were also available with GM Hydra-Matic. Mercedes-Benz subsequently devised a four-speed fluid coupling transmission that was similar in principle to Hydra-Matic, but did not share the same design.

     The first torque converter automatic, Buick's Dynaflow, was introduced for the 1948 model year. It was followed by Packard's Ultramatic in mid-1949 and Chevrolet's Powerglide for the 1950 model year. Each of these transmissions had only two forward speeds, relying on the torque converter for additional gear reduction. In the early 1950s Borg-Warner developed a series of three-speed torque converter automatics for American Motors Corporation, Ford Motor Company, Studebaker, and several other manufacturers in the US and other countries. Chrysler was late in developing its own true automatic, introducing the two-speed torque converter PowerFlite in 1953 and the three-speed TorqueFlite in 1956.

     By the late 1960s most of the fluid-coupling four-speeds and two-speed transmissions had disappeared in favor of three-speed units with torque converters. By the early 1980s these were being supplemented and eventually replaced by overdrive-equipped transmissions providing four or more forward speeds. Many transmissions also adopted the lock-up torque converter (a mechanical clutch locking the torque converter impeller and turbine together to eliminate slip at cruising speed) to improve fuel economy.

     As the engine computers became more and more capable, even more of the valve body's functionality was offloaded to them. These transmissions, introduced in the late 1980s and early 1990s, remove almost all of the control logic from the valve body, and place it in into the engine computer. (Some manufacturers use a separate computer dedicated to the transmission but sharing information with the engine management computer.) In this case, solenoids turned on and off by the computer control shift patterns and gear ratios, rather than the spring-loaded valves in the valve body. This allows for more precise control of shift points, shift quality, lower shift times, and (on some newer cars) semi-automatic control, where the driver tells the computer when to shift. The result is an impressive combination of efficiency and smoothness. Some computers even identify the driver's style and adapt to best suit it.


AUTOMATIC TRANSMISSION MODES
     In order to select the mode, the driver must move a gear shift lever which can be located on the steering column or on the floor next to the driver. In order to select gears/modes the driver must push a button in (called the shift lock button) or pull the handle (only on column mounted shifters) out.

     

Automatic Transmissions have various modes depending on the model and make of the transmission. Some of the common modes are:

     Park (P) – This selection mechanically locks the transmission, restricting the car from moving in any direction. A pin prevents the transmission from moving forward (although wheels, depending on the drive train, can still spin freely), it is recommended to use the hand brake (or emergency brake) because this actually locks the wheels and prevents them from moving, and increases the life of the transmission and the park mechanism. A car should be allowed to come to a complete stop before setting transmission into park to prevent damage. Park is one of only two selections in which the car can be started. In some cars (notably those sold in the US), the driver must have the foot brake depressed before the transmission can be taken out of park.

     Reverse (R) – This puts the car into the reverse gear, giving the ability for the car to back up. In order for the driver to select reverse they must come to a complete stop, and push the shift lock button in and select reverse. Not coming to a complete stop can cause severe damage to the transmission.

     Neutral/No gear (N) – This disconnects the transmission from the wheels so the car can move freely under its own weight. This is the only other selection in which the car can be started.

     Drive (D) – This allows the car to move forward and accelerate through a range of gears. The number of gears a transmission has depends on the model, but they can commonly range from 3, 4 (the most common), 5, 6 (found in VW/Audi Direct Shift Gearbox), and 8 in the new model of Lexus cars.

     D4 – In Honda and Acura automatics this mode is used commonly for highway use (as stated in the manual) and uses all 4 forward gears.

      D3 – This is also found in Honda and Acura automatics and only uses the first 3 gears and according to the manual it is used for stop & go traffic such as city driving.

      + - and M – This is the manual selection of gears for automatics with Tiptronic. The driver can shift up and down at their will.

      Second (2 or S) – This mode limits the transmission to the first two gears, or more commonly locks the transmission in second gear. This can be used to drive in adverse conditions such as snow and ice, as well as climbing or going down hills in the winter time.

     First (1 or L) – This mode locks the transmission in first gear only. It will not accelerate through any gear range. This, like second, can be used during the winter season, or towing.

Some cars when put into D will automatically lock the doors or turn on the daytime running lights.