"If my sat nav shows I'm going 4/5 miles an hour slower than my car speedometer says, which do I believe?" - Asked by car owners everywhere.
Many drivers have reported this exact phenomenon in recent years, as satnav has become more prevalent in cars, and therefore the situation is really common to just about every car on the market. But why is that this the case? Surely car manufacturers can make their speedos accurate to the precise mph or km/h you're travelling at?
How a car speedometer works
Speed is the measurement of distance over time. But a car speedometer doesn't measure how briskly you travel from Point A to Point B. Car speedos usually work by measuring the rotation of the car's driveshaft, axle or wheel. They then use some basic maths to extrapolate that rotation and determine how briskly you're travelling. it is a very similar concept to a bicycle speedometer.
However, if the diameter of the wheel/tyre alters, the extrapolation calculation is going to be incorrect. for instance, if the diameter will increase if you set new tyres on the car (more tread, which wears down over thousands of miles) or increase the tyre pressure. this suggests that, for every revolution of the wheel, the car is travelling further, meaning your speed is bigger.
If the diameter decreases (eg - worn tyres, less air within the tyres, a special brand of tyre with slightly different dimensions, more load within the car weighing it down and compressing the tyres), then the car are going to be travelling slower for every revolution of the wheel.
The margin of error during a car speedometer
The differences in wheel diameter resulting from the above circumstances might be tiny (maybe a couple of millimetres), but at 30mph your car wheels are rotating 6-7 times every second, so it can quickly make a difference of a couple of miles per hour. This margin for error is taken under consideration in how the law is applied, and the way manufacturers calibrate their car speedos.
How a satnav speedometer works
Satnav measures your car's speed by the actual distance over time using GPS satellite tracking. It repeatedly locates your exact position on earth via satellite and calculates how far you've got travelled, then divides by the time it took for you to travel that distance. Satnav accuracy is decided by satellite signal quality and is unaffected by your car's tyres. Many satnavs are unable to account for changes in the vertical direction, so could also be less accurate if you're travelling up or down a steep hill. they're also inherently more accurate at higher speeds, as a bigger distance over time reduces rounding errors.
Some factory satnav systems also will use data from the car to integrate with the GPS signal to enhance overall accuracy.
The law for car speedometers within the UK
The UK law is predicated on the EU standard, with some minor changes. A speedo must not ever show but the particular speed, and must not ever show quite 110% of actual speed + 6.25mph. So if your actual speed is 40mph, your speedo could legally be reading up to 50.25mph but never but 40mph. Or to place it differently, if your speedo is reading 50mph, you will not be doing quite 50mph but it's possible you would possibly actually only be travelling at 40mph.
To ensure that they suit the law and confirm that their speedometers are never showing but actual speed, car manufacturers will normally deliberately calibrate their speedos to read 'high' by a particular amount. So that's why your car speedometer reads above your satnav.
Stuart Masson is founder and owner of The Car Expert, a London-based independent and impartial car buying agency for anyone looking to shop for a replacement or used car.
Originally from Australia, Stuart has had a passion for cars and therefore the automotive industry for nearly thirty years, and has spent the last seven years working within the automotive retail industry, both in Australia and in London.
Stuart has combined his extensive knowledge of all things car-related together with his own experience of selling cars and delivering high levels of customer satisfaction to bring a singular and private car buying agency to London. The Car Expert offers specific and tailored advice for anyone trying to find a replacement or used car in London.