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A speedometer measures the speed of a vehicle. Traditional automotive speedometers are driven by a flexible, sleeved cable that is rotated by a set of small gears in the tail shaft of a transmission. The speedometer itself is two rotating, barrel-shaped magnets. One magnet is connected the sleeved cable, and the other is connected to the speedometer needle. These magnets are calibrated such that a given revolution speed of the flexible cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tires. The speedometer mechanism often also drives an odometer. In Australia Law allowes a 10% error for measurement. However in some states (such as Victoria and NSW) speed camera’s tolerate less than 10% making it hard to accurately measure your speed.
Speedometer Cable | Wires

The speedometer cable is connected either to the gearbox output shaft, transmission shaft, or differential. Speed and record mileage is measured by the rotation of these shafts. This information is sent back through the cable where it is recorded on the speedometer. The speedometer and odometer are driven by a cable housed in a flexible casing, which is connected to a gear in the transmission. Speedometer cables break as the result of age, lack of lubrication, or because the cable casing has sharp bends. It may also break from too much friction in the speedometer head.