A transmission shifts gears based on vehicle speed and accelerator input (how far down you push your car’s pedal) to keep the engine’s RPM, or “revolutions per minute” as low as possible.
This has two advantages:
A transmission also allows you to use your vehicles engine energy when necessary. When driving slowly, you can keep your engine in a lower gear and avoid damaging it. When you need to move faster, you can shift into a higher gear. As a result, the transmission allows you to take advantage of more of your cars power by rotating the wheels more quickly.