A car speedometer is an instrument that provides the driver with instantaneous visual readings of speed which is usually measured in Kilometer per hour (km/hr). Traditional speedometers used gears and wires to determine speed, while most modern vehicles use speed sensors for the same.

There are several common causes for a speedometer to stop working. Typically, these malfunctions are caused by, a broken gear in the speedometer system, a speed sensor issue or a faulty engine control unit (ECU).

A malfunctioning speedometer may also be due to damaged wiring. Wiring is vulnerable to water damage and can cause fuses to blow. If the problem appears during wet weather especially or if the car has been exposed to excessive water, there is a good chance that this could be the issue.


In older cars a break in the cable that connects the transmission to the speedometer is the most common cause. Cars produced after 1990 are usually equipped with speed sensors, which may crash and cease to transmit speed readings to the speedometer. A more serious problem could be a faulty speedometer head, which needs expert diagnosis.

A faulty speed sensor is the most likely culprit in the case of a malfunctioning speedometer. Often covered under warranty, the speed sensor is located near the front axle and calculates the speed of the vehicle.


Lastly, many automobile problems can be traced back to a malfunctioning ECU, which is the main computer that manages many of the car’s processes. If the ECU is not working, it is possible that it is preventing the speed sensor from reporting the speed to the speedometer. A broken ECU breaks this link between the two components and could cause an issue.

Speedometers are calibrated according to the radius and diameters of the factory-fitted tires of your car. These determinants can change, if you get custom tires which are larger, or if they are of different dimensions. The rate at which your tires cover ground changes and if the speedometer is not calibrated accordingly, it can show a faulty reading.

Credit: Automedics

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