From the Wikipedia Article on Transmission Mechanics:

Often, a transmission will have multiple gear ratios (or simply "gears"), with the ability to switch between them as speed varies. This switching may be done manually (by the operator), or automatically. Directional (forward and reverse) control may also be provided. Single-ratio transmissions also exist, which simply change the speed and torque (and sometimes direction) of motor output.

Shifting gears is part of the standard operation of a vehicle to maintain the engine's optimum power output, fuel efficiency, and overall health. However, operators can shift gears manually in both manual and automatic transmissions equipped vehicles, for safety reasons, when operating on slippery terrain or steep downgrades. Downshifting an automatic is an example of a question with this tag, and the question covers the mechanical effects of a certain type or style of vehicle operation.

Additionally, there are other example questions that involve shifting early, late, shifting when cold, gear grinding when cold, and other questions related to problems with transmission gear shifting.

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