Speedometer

SpeedometerNormally, depending on how modern is your car, its dashboard is stuffed full of various sensors and readers of current vehicle status. Generally, each of sensors indicates emergency or malfunction of a subassembly, informs you also that you should visit a car diagnostic centre to supplement or replace a component. Although dashboards may differ from each other to some extent, each of them has one common component – a meter known also as a speedometer.

Speedometer

Missing speedometer prevents safe, and first of all, lawful driving on all roads. Just it allows you to check your driving speed. The principle of operation of modern speedometer consists in converting mechanical revolutions of the drive shaft into electrical pulses.   In older vehicles a flexible shaft rotating inside a gearing case is used. This shaft moves the meter pointer, thus indicating the driving speed of your car.  All meters of such type require an appropriate calibration that is done in such a way so that a slightly higher speed than actual one is indicated. Obviously, this is to enhance safety of drivers, as an overestimation prevents you to exceed the allowable limit speed. In both cases the end of the meter shaft is placed on the rear part of the drive shaft. In the case of electronic meter its indications are represented by a pulse travelling to the computer where it is converted into digital form.
The accuracy of speedometer readout or indication is affected also by air pressure in tyres.  In this point it should be noted that too low tyre pressure speeds up tyre wear and increase fuel consumption.

See more at: http://www.auto24parts.com/en_GB/c/SPEEDOMETER/706