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Speedometer

A speedometer is a truck parts device that measures the instantaneous speed of a land vehicle.

Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards.Speedometers for other vehicles 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.

Electronic

Now in 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.

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 position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms.

Error

Speedometers are not totally accurate, and most speedometers have tolerances of some 10% plus or minus due to wear on tires as it occurs. Additional sources of error are tire diameter variations due to temperature, pressure, vehicle load, and nominal tire size.
Modern speedometers are said to be accurate within 10% but as this is legislated accuracy, this may not be entirely correct.

This can make it difficult to accurately stay on the speed limits imposed; most countries allow for this known variance when using RADAR to measure speed, although levels of some 3 km/h or 3% are also used in areas of tough enforcement. This causes many arguments due to motorists complaining that they were not doing the speed as reported. Revenue is being increasingly blamed for these stricter measures.

There are strict United Nations standards in place but it seems not being enforced leaving this matter in limbo for many countries. Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the

percent error = 100x("standard diameter"/"new diameter" - 1)
Nearly all tires now have their size shown as "T/A_W" on the side of the tire (See: Tire code), and the tire's
diameter in inches = TxA/1270 + W.
For example, a standard tire is "185/70R14" with diameter = 185x70/1270 + 14 = 24.20 in. Another is "195/50R15" with 195x50/1270 + 15 = 22.68 in. Replacing the first tire (and wheels) with the second (on 15" wheels), a speedometer reads 24.19/22.68 = 1.0670 times the correct speed or 6.7% too high.

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