Editing Speedometer (section)

== Operation ==
===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.

===Electronic===
{{see also|Vehicle speed sensor}}
Many modern speedometers are [[electronics|electronic]]. In designs derived from earlier eddy-current models, a rotation sensor mounted in the transmission delivers a series of electronic pulses whose frequency corresponds to the (average) rotational speed of the [[drive shaft|driveshaft]], and therefore the vehicle's speed, assuming the wheels have full traction. The sensor is typically a set of one or more magnets mounted on the output shaft or (in transaxles) differential crown wheel, or a toothed metal disk positioned between a magnet and a [[Hall effect|magnetic field sensor]]. As the part in question turns, the magnets or teeth pass beneath the sensor, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring.<ref name="how"/> Alternatively, particularly in vehicles with multiplex wiring, some manufacturers use the pulses coming from the ABS wheel sensors which communicate to the instrument panel via the [[CAN Bus]]. Most modern electronic speedometers have the additional ability over the eddy current type to show the vehicle's speed when moving in reverse gear.

A computer converts the pulses to a speed and displays this speed on an electronically controlled, analogue-style needle or a [[digital display]]. Pulse information is also used for a variety of other purposes by the [[Engine Control Unit|ECU]] or full-vehicle control system, e.g. triggering ABS or traction control, calculating average trip speed, or increment the [[odometer]] in place of it being turned directly by the speedometer cable.

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 endeavours 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.

====Virtual====
Virtual speedometers typically approximate speed based on distance traveled over time with the help of a satellite radio navigation system, such as [[GPS]]. Virtual speedometers tend to be less accurate than their analog counterparts and are affected by environmental factors such as weather conditions, terrain, and obstructions in the way of the signal.

==== Bicycle speedometers ====
{{main|Cyclocomputer}}
Typical [[bicycle]] speedometers measure the time between each wheel revolution and give a readout on a small, handlebar-mounted digital display. The sensor is mounted on the bike at a fixed location, pulsing when the spoke-mounted magnet passes by. In this way, it is analogous to an electronic car speedometer using pulses from an ABS sensor, but with a much cruder time/distance resolution – typically one pulse/display update per revolution, or as seldom as once every 2–3 seconds at low speed with a {{convert|26|in|mm|0|adj=on}} wheel. However, this is rarely a critical problem, and the system provides frequent updates at higher road speeds where the information is of more importance. The low pulse frequency also has little impact on measurement accuracy, as these digital devices can be programmed by wheel size, or additionally by wheel or tire circumference to make distance measurements more accurate and precise than a typical motor vehicle gauge. However, these devices carry some minor disadvantages in requiring power from batteries that must be replaced every so often in the receiver (and sensor, for wireless models), and, in wired models, the signal is carried by a thin cable that is much less robust than that used for brakes, gears, or cabled speedometers.

Other, usually older bicycle speedometers are cable driven from one or other wheel, as in the motorcycle speedometers described above. These do not require battery power, but can be relatively bulky and heavy, and may be less accurate. The turning force at the wheel may be provided either from a gearing system at the hub (making use of the presence of e.g. a hub brake, cylinder gear, or dynamo) as per a typical motorcycle, or with a friction wheel device that pushes against the outer edge of the rim (same position as rim brakes, but on the opposite edge of the fork) or the sidewall of the tire itself. The former type is quite reliable and low maintenance but needs a gauge and hub gearing properly matched to the rim and tire size, whereas the latter requires little or no calibration for a moderately accurate readout (with standard tires, the "distance" covered in each wheel rotation by a friction wheel set against the rim should scale fairly linearly with wheel size, almost as if it were rolling along the ground itself) but are unsuitable for off-road use, and must be kept properly tensioned and clean of road dirt to avoid slipping or jamming.