There is no engine component more complex and essential than a car's transmission.

Transmissions are responsible for shifting gears (automatically or manually) to dynamically change the speed-torque ratio in order to deliver power to the wheels in an efficient manner based on the speed of the vehicle. The reason for this is that engines output a high rotational speed which cannot be used for low speeds or to initiate. Using gear ratios, a transmission reduces the rotational speed and increases the torque (or power to the wheels) in the process.