What does a car's speedometer measure?

A car’s speedometer measures and displays its speed. It tells the driver how quickly the vehicle is moving, usually in miles per hour, kilometers per hour, or both.

Traditional mechanical speedometers receive information from the vehicle’s transmission or wheels. A rotating cable drives a magnet and pointer inside the gauge, with the pointer’s position increasing as the vehicle moves faster. Modern cars generally use electronic sensors, often connected to the transmission, wheel-speed system, or vehicle computer, to calculate speed and send it to a digital or analog display.

A speedometer is different from an odometer, which records the total distance a vehicle has traveled. It is also different from the tachometer, which shows engine speed in revolutions per minute. Speedometers can be slightly inaccurate because of tire size, tire wear, calibration, or legal tolerances, so the displayed speed may not exactly match GPS speed.