What is the name of the device that measures a car's speed?

The device that measures a car's speed is the speedometer. It displays the vehicle's instantaneous speed, usually in miles per hour, kilometers per hour, or both.

Early automotive speedometers commonly used a flexible cable connected to the transmission or a wheel. As the cable rotated, it drove a magnet inside the gauge; magnetic drag moved the pointer against a calibrated spring. Modern cars generally use electronic sensors that count drivetrain or wheel rotations and send pulses to the instrument cluster.

A speedometer is often confused with an odometer or tachometer. The odometer records the distance a vehicle has traveled, while the tachometer measures engine speed in revolutions per minute. Speedometers can also differ slightly from true road speed because tire size, tire wear, gearing, and legal calibration tolerances affect the reading. The modern speedometer developed in the early twentieth century, although Josip Belušić demonstrated an electric speed-measuring device in 1889.