speedometer

A speedometer is a gauge that displays the instantaneous speed of a vehicle. It is typically part of the instrument cluster and is used to monitor adherence to speed limits and to inform driving decisions. Most road-going vehicles show speed in mph or km/h, and some provide both.

There are two broad families: mechanical and electronic. Mechanical speedometers use a drive cable from the transmission to move a pointer via gearing or magnetic coupling. Electronic speedometers use a vehicle speed sensor to produce pulses, with a module converting them to a display.

Accuracy depends on tire size, wear, and calibration. Because readings come from wheel rotation, changes in tire diameter alter indicated speed and odometer distance. Regulatory standards commonly require the indicated speed to remain within a defined tolerance of actual speed. If readings drift or become erratic, causes include worn cables, failed sensors, or the need for recalibration after tire changes.

Related instruments include GPS-based speed readouts and aviation speed indicators, which use different principles. Speedometers may switch units or warn when speeding; wheel speed sensors and instrument clusters also support ABS and stability control.