Every car on the road has a degree of speedometer error.
In a standard setup where the car has not been modified the speedo will always read a speed that is slightly higher than the actual speed that the vehicle is travelling.
Manufacturers deliberately design the system in this way to ensure that the speedo never reads a speed lower than what the car is actually travelling.
This is done for both safety reasons and so that the manufacturer cannot be liable for speeding fines incurred by the driver.
The degree of speedo error in a car depends on a number of factors, but it can be quite high in some cases – especially on older vehicles.
This is due to the fact that older vehicles use a cable to measure the car’s speed, and the cable wears out and stretches over many years of use.
In one case we measured the speed of a 30 year-old car using GPS and found it to only be travelling at 80km/h when the speedo indicated a speed of 100km/h.
The biggest thing that affects speedo error is wheel diameter.
The way that speedometers work is by having a pre-programmed figure for the diameter of the wheels on the car and combining that with how quickly the wheels are making one full rotation.
The system uses those two pieces of information to calculate how fast the car is travelling.
If you change the wheels on your car then the new wheels are likely to be a different diameter which will mean that the car is calculating your speed incorrectly.
There are international agreements that govern the degree of speedo error, but the variance can still be fairly large – even in new cars – where your speedo could be reading 120km/h and you are actually only travelling at 110km/h.
So next time you think you’re sitting at the speed limit on the highway and the car beside you overtakes you without getting flashed – you may know why.