Speedometer gauge on a car, showing the speed of the vehicle in miles and kilometre per hour on the out– and inside respectively. Also shown is the gear indicator, as well as the tachometer, which displays the engine rotations per minute.
A speedometer measures the instantaneous speed of a vehicle.
Traditional automotive speedometers are driven by a flexible, sleeved cable that is rotated by a set of small gears in the tail shaft of a transmission. The speedometer itself is two rotating, barrel-shaped magnets. One magnet is connected the sleeved cable, and the other is connected to the speedometer needle. These magnets are calibrated such that a given revolution speed of the flexible cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tires. The speedometer mechanism often also drives an odometer.
The speedometer was invented by Josip Belušić of Croatia in 1888 and was electric. Modern speedometers are electronic. A rotation sensor, usually mounted on the rear of the transmission, delivers a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. A computer converts the pulses to a speed and displays this speed on a digital display or an electronically-controlled, analog-style needle, the latter of which is more common. Pulse counts may also be used to increment the odometer.
As of 1997, federal standards in the United States allowed a maximum 5% error on speedometer readings (per "Auto Tutor", American Automobile Association of California magazine, Oct. 17, 1997). Aftermarket modifications, such as different tire and wheel sizes or different differential gearing, can cause speedometer inaccuracy.
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Speedometers for other craft have specific names and use other means of sensing speed. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator.