Editing Speedometer (section)

===Mechanical===
Many speedometers use a rotating [[Flexible shaft|flexible cable]] driven by gearing linked to the vehicle's [[Transmission (mechanics)|transmission]]. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel.

Some early mechanical speedometers operated on the governor principle where a rotating weight acting against a spring moved further out as the speed increased, similar to the [[Governor (device)|governor]] used on steam engines. This movement was transferred to the pointer to indicate speed. 

This was followed by the Chronometric speedometer where the distance traveled was measured over a precise interval of time (Some Smiths speedometers used 3/4 of a second) measured by an escapement. This was transferred to the speedometer pointer. The chronometric speedometer is tolerant of vibration and was used in motorcycles up to the 1970s.

When the vehicle is in motion, a speedometer gear assembly turns a speedometer cable, which then turns the speedometer mechanism itself. A small permanent magnet affixed to the speedometer cable interacts with a small aluminium cup (called a ''speedcup'') attached to the shaft of the pointer on the analogue speedometer instrument. As the magnet rotates near the cup, the changing magnetic field produces [[eddy current]] in the cup, which itself produces another magnetic field. The effect is that the magnet exerts a [[torque]] on the cup, "dragging" it, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.<ref name="how"/>

The pointer shaft is held toward zero by a fine [[torsion spring]]. The torque on the cup increases with the speed of rotation of the magnet. Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. The cup and pointer will turn until the torque of the eddy currents on the cup are balanced by the opposing torque of the spring, and then stop. Given the torque on the cup is proportional to the car's speed, and the spring's deflection is proportional to the torque, the angle of the pointer is also proportional to the speed, so that equally spaced markers on the dial can be used for gaps in speed. At a given speed, the pointer will remain motionless and point to the appropriate number on the speedometer's dial.

The return spring is [[calibration|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 tail shaft gears that drive the flexible cable, the final drive ratio in the [[differential (mechanics)|differential]], and the diameter of the driven [[tires]].

One of the key disadvantages of the eddy current speedometer is that it cannot show the vehicle speed when running in reverse gear since the cup would turn in the opposite direction – in this scenario, the needle would be driven against its mechanical stop pin on the zero position.