Which should I trust - my speedometer or my sat nav?

Q. My sat nav generally indicates a road speed lower than that displayed on my car’s speedometer. At 70mph the difference between the two can be as much as 5mph. Which should I trust? BC, via email

A. Your speedo and your sat nav calculate speed in different ways. Speedometers usually work by continuously counting turns of a driveshaft, the gearbox or a wheel, and relating them to the rolling circumference of the car’s tyres to give the road speed.

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Because the circumference of the wheels and tyres can change — with wear, for instance, or the aftermarket fitting of larger wheels and tyres — speedometers have to allow for a margin of error. Legally they may overstate a vehicle’s speed by up to 10% but may not understate it. Your speedo will therefore nearly always indicate a road speed that’s slightly higher than the one you are doing.

Sat navs, though, calculate your speed by constantly updating your exact location via global positioning satellites (GPS), measuring the distance travelled since your last co-ordinate and comparing it with the time taken. On a straight and level road, at constant speed, the GPS-derived speed reading will be accurate.

However, there is usually some lag in the reading, and accuracy can be affected by GPS signal quality, bends in the road, inclines and speed changes. Fortunately, modern factory-fitted systems also use data from the car itself, integrating it with the GPS information so as to enhance the accuracy.

So, in general, your sat nav is likely to be more accurate than the car’s speedometer, but for safe driving, rely on the latter because it will never understate your speed.

LADY DRIVER
Emma Smith is a journalist specialising in consumer issues and is a regular Driving contributor – read more from Emma here.

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