Ever found yourself cruising down the road, only to realise your car's speedometer is stuck in limbo? Find out why your speedometer readings are inaccurate in this article.
What is a speedometer?
The speedometer is the small gauge on the dashboard that indicates the speed the car is travelling at in miles or kilometres per hour. They can be analogue or digital gauges, similar to analogue and digital clocks.
Many new cars have fancy electronic speedometers. From the moment you put your foot down on the accelerator pedal, the vehicle’s speed monitoring system gets a signal from the transmission output shaft speed sensor, which is then converted into the actual speed reading by the onboard computer and displayed via the speedometer.
Despite the advancement of technology, a large number of cars still use a traditional mechanical gauge, which operates via a cable instead of a sensor. The speed is measured using electromagnetism. The bendy cable connects the output shaft that turns the wheels to the speedometer itself.
As the shaft rotates, the cable turns a magnet inside the gauge’s housing. The magnet is enclosed in a small metal cup, which is also able to rotate. The cup is connected to the speed dial’s pointer which moves up and down thanks to the magnetic force created.
How accurate is a car speedometer?
The speedometer doesn’t technically measure the speed you travel from point A to point B. Instead, it measures the speed of rotation to calculate this. The problem here is that changes to the diameter of the wheel or tyre could affect the accuracy of this conversion.
For example, if the air pressure is too high, the diameter will increase. This means that each rotation will cover a further distance, affecting the speed. If the diameter decreases, the same principle applies except the wheel will cover a shorter distance.
You may have noticed that the speed dial is showing a slightly higher speed than your sat nav or mobile device. While speedometers are forbidden from showing reading below the actual speed, the readings can be slightly higher. Most manufacturers typically calibrate the dial at 5% above the actual speed of travel to account for errors.
Why is my speedometer not working?
There could be a number of possible reasons why your speedometer is not working properly. Apart from the issue mentioned above, the external conditions and condition of the parts can increase or decrease the likelihood of a malfunction.
Possible causes:
Wear or damage
False readings can be caused by worn or damaged components, such as the sensor, cables , wires, and gears. Check the parts for visible signs of damage and replace them if necessary. Depending on the vehicle model, the speedometer cable should last about 2 to 5 years, while some last much longer.
Calibration issues
Speedometers are often calibrated to be accurate at a certain speed. However, they may not be perfectly calibrated across the entire speed range. Additionally, if you modify your vehicle, i.e., by changing the gear ratio or installing aftermarket wheels, the speedometer may need to be recalibrated by a mechanic.
Sensor malfunctions
If the speedometer has stopped working and started again, there is a high chance that it is due to a faulty sensor. The erratic readings are usually a result of an interrupted or bad sensor signal. Common causes include a damaged sensor, corroded connections, broken wires, and contamination.
ECU faults
ECU problems can affect the calculation of the speed in electronic systems, resulting in incorrect speedometer readings.
How to fix speedometer
The first thing to do is identify the root cause of the problem. As outlined above, you may be required to replace certain parts, such as the speedometer cable or sensor wiring, or the whole unit.
It may be possible to restore the sensor by cleaning it to remove buildups of dirt and debris which can disrupt the signal. If the speedometer needs to be recalibrated, it is best to seek professional assistance as some are more complicated to fix than others.
Furthermore, make sure to buy parts that are compatible with your vehicle. For example, if you’re looking for a Citroen C4 speedometer cable online, use vehicle specific data such as the Citroen’s OEM part number or registration number for more accurate results.
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