Speedometer, instrument for indicating the speed of a motor vehicle. The common type works by magnetic induction. A circular, permanent magnet is rotated by a flexible cable geared to the transmission. The rotating magnetic field induces a magnetic field in an aluminum cup, thus tending to turn it in the same direction as the magnet. This torque, proportional to the speed of rotation, is opposed by a spiral spring. The angle through which the cup turns against the spring measures the speed. The speedometer is usually coupled with an odometer, a counting device geared to the magnet, that registers the distance traveled.