A vehicle’s speedometer is a gauge or meter that measures the speed of a motor vehicle. Is does this by measuring the speed at which the wheels are rotating and communicating this to the speedometer head, which then displays this as the vehicle’s speed.

There are two principal means by which the speedometer system communicates the necessary information to the speedometer head. The first of these is by way of a speedometer cable. This is a lengthy, flexible cable that comprises a collection of wires connected to the drive shaft, which turns as the wheels do. The rotation of the drive shaft causes the cable to turn which causes a magnet within a cup inside the speedometer head to turn. This causes currents that produce motion which, in turn, causes the speedometer dial to move and demonstrate the vehicle’s speed.

The second means of transmitting information to the speedometer head is electronically, by means of a speedometer transducer. The speedometer transducer is normally attached to the vehicle’s transmission and is an electronic pulse generator. It sends signals to the car’s computer as to how quickly the wheels are turning and this information is translated by the computer into the speed that is displayed by the speedometer.

In either type of speedometer system there are various components that are susceptible to failure but, fortunately, all of these are easily replaced.