A speedometer is a vehicle instrument that measures the instantaneous speed.

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 early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel.

The most common form of speedometer relies on the interaction of a small permanent magnet affixed to the rotating cable with a small aluminum cup affixed to the shaft of the pointer. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet ‘drags’ the cup and thus the speedometer pointer in the direction of its rotation with no mechanical connection between them…

The eddy current speedometer has been used for over a century and is still in widespread use. Until the 1980s and the appearance of electronic speedometers it was the only type commonly used.

Originally patented by a German, Otto Schulze on 7 October 1902, it uses a rotating flexible cable usually driven by gearing linked to the tail shaft (output) of the vehicle’s transmission. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel.

A small permanent magnet affixed to the rotating cable interacts with a small aluminum cup (called a speedcup) attached to the shaft of the pointer on the analogue instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet “drags” the cup, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.

The pointer shaft is held toward zero by a fine spring. The torque on the cup increases with the speed of rotation of the magnet (which is driven by the car’s transmission.) Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. When the torque due to the eddy currents in the cup equals that provided by the spring on the pointer shaft, the pointer will remain motionless and pointing to the appropriate number on the speedometer’s dial.

The return spring is calibrated such that a given revolution speed of the 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.
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Many 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. The sensor is typically a toothed metal disk positioned between a coil and a magnetic field sensor. As the disk turns, the teeth pass between the two, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring.[1] Alternatively, some manufactures rely on pulses coming from the ABS wheel sensors.

A computer converts the pulses to a speed and displays this speed on an electronically-controlled, analog-style needle or a digital display. Pulse counts may also be used to increment the odometer.

Another early form of electronic speedometer relies upon the interaction between a precision watch mechanism and a mechanical pulsator driven by the car’s wheel or transmission. The watch mechanism endeavors to push the speedometer pointer toward zero, while the vehicle-driven pulsator tries to push it toward infinity. The position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms.

Changing a car’s tire size can throw off a speedometer’s accuracy.

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