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An automatic transmission (commonly abbreviated as "AT") is an gearbox that can change gear ratios automatically as the vehicle moves, thus freeing the driver from having to shift gears manually (similar but larger devices are also used for railroad locomotives).

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.

Recently manufacturers have begun to make continuously variable transmissions commonly available (earlier models such as the Subaru Justy did not popularize CVT). These designs can change the ratios over a range rather than between set gear ratios. Even though CVTs have been used for decades in two-wheeled scooters and in a few cars (e.g. DAF saloons and the Volvo 340 series that succeeded them, and later the Subaru Justy), the technology has recently gained greater acceptance among manufacturers and customers.

Early Combine Harvesters used a version of CVT, to vary the input speed to the gear box using a Variable ratio belt drive as on the Daf car, so a variable speed range is available in each gear allowing constant engine speed to drive the threshing mechanism.

The other type that has become more comon is the TorqueConverter, or Hydrostatic transmissions in machines such as Combines, Excavators, Forklifts, and other plant.

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