“If my satnav shows I’m going four to five miles per hour slower than my car speedometer says, which do I believe?” Asked by Tabitha, TCE reader.
Having recently bought a used car, Tabitha began to notice that there was a consistent discrepancy between the speed showing on her speedometer and that on her portable satellite navigation unit. At most speeds, the speedo reading was several mph faster than her satnav unit was indicating.
Many other drivers have reported the same thing over the years, and this situation is actually common to nearly every car on the market. But why is this the case? Surely car manufacturers can make their speedos accurate to the precise mph or km/h you are travelling at?
How a car speedometer works
Speed is the measurement of distance over time. But a car speedometer doesn’t actually measure how fast you travel from Point A to Point B. Car speedos usually work by measuring rotation of the car’s driveshaft, axle or wheel. They then use some basic maths to extrapolate that rotation and determine how fast you are travelling. It’s a very similar concept to a bicycle speedometer.
However, if the diameter of the wheel/tyre alters, the extrapolation calculation will be incorrect. For example, the diameter will increase if you put new tyres on the car (more tread, which wears down over thousands of miles) or increase the tyre pressure. This means that, for each revolution of the wheel, the car is travelling further, meaning your speed is greater.
If the diameter decreases (eg – worn tyres, less air in the tyres, a different brand of tyre with slightly different dimensions), then the car will be travelling a shorter distance for each revolution of the wheel, therefore you will be going slower.
Margin of error in a car speedometer
The differences in wheel diameter resulting from the above circumstances could be tiny (maybe a few millimetres), but at 30mph your car wheels are rotating 6-7 times every second, so it can quickly make a difference of a few miles per hour.
This margin for error is taken into account in how the law is applied, and how manufacturers calibrate their car speedos.
How a satnav speedometer works
Satellite navigation units (either portable or integrated into the car) calculate your car’s speed by measuring actual distance travelled over time using GPS satellite tracking.
They repeatedly locate your exact position on earth via satellite and calculate how far you have travelled, then divide by the time it took for you to travel that distance. Satnav accuracy is determined by satellite signal quality and is unaffected by your car’s tyres.
Many satnavs are unable to account for changes in vertical direction, so they may be less accurate if you are travelling up or down a steep hill. They are also inherently more accurate at higher speeds, as a larger distance over time reduces rounding errors, but a satnav will usually be much closer to a car’s true speed than the speedometer.
Some factory satnav systems will also use data from the car to integrate with the GPS signal to improve overall accuracy.
The law for car speedometers in the UK
So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.
To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount. As your satnav is not the designated device by which a car’s speed is measured, it does not need to incorporate any fudge factoring.
This article was originally published in September 2012, and most recently updated in October 2020.