Speedometer

A speedometer measures and displays the speed of a moving vehicle or other object. Vehicle speedometers commonly show speed in kilometers per hour or miles per hour.

Eddy-current speedometer: A mechanical speedometer that uses a rotating magnet to induce currents in a nearby metal cup, moving a pointer against a spring. This classic design converts rotation of a vehicle’s transmission into pointer movement.
Cruise control: A vehicle system that automatically maintains a selected speed without continuous accelerator input. It uses speed measurements as feedback to regulate vehicle motion.
Speed: The rate at which an object covers distance, expressed as distance per unit time. A speedometer displays an estimate of this physical quantity.
Speedometer error: The difference between a speedometer’s displayed speed and the vehicle’s actual speed. Errors affect how drivers interpret speed and how manufacturers design displays.
Electronic speedometer: A speedometer that calculates speed from electrical pulses generated by a vehicle sensor. It replaces the rotating cable and magnetic cup with sensor pulses and electronic calculation.
Speed limit: A legally established maximum speed for vehicles on a road or in an area. Drivers compare the displayed speed with the applicable legal limit.
Velocity: The rate of change of position in a specified direction. A conventional speedometer gives speed, not the direction required to specify velocity.
Speedometer laws: Legal requirements governing vehicle speed indication, including accuracy and display standards. Regulations determine permissible speedometer error and required units or visibility.
Wheel speed sensor: A sensor that detects the rotational speed of a vehicle wheel, usually for braking and stability systems. Wheel rotation can provide the input from which an electronic speedometer estimates vehicle speed.
Speed camera: A camera system that detects vehicle speed and records evidence of suspected speeding. Enforcement systems compare measured speed against legal limits.

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