A speedometer is a car parts device that measures the instantaneous
speed of a land vehicles.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.
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.
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.
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.
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 (three)km/h or 3% are also used in areas of tough
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 like this, a standard tire is "185/70R14" with diameter = 185x70/1272
+ 14 = 24.22 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 = 1.0671 times the correct
speed or 6.7% too high.