Have you ever noticed a discrepancy between your vehicle's speedometer and the speed stated by your GPS system? If so, you're in good company.

 

But which is correct? Your speedometer or your GPS? In nearly all such cases, the speedometer is incorrect - reporting a higher speed than your car is actually travelling at.

You might imagine that manufacturers ensure their speedometers are as accurate as possible. So why the discrepancy?

 

A car's speedometer works by measuring the rotations of the driveshaft, axle or wheel - rather than the speed you actually travel from one point to another. The car takes the recorded rotations, applies a healthy dose of mathematics and spits out a speed.

 

But the accuracy of a speedometer depends on the wheels remaining the same diameter as they were when they rolled off the production line. If the tyres or wheels are changed, this will change the speedometer reading. A larger diameter will result in a faster recorded speed. Fitting smaller tyres or having under inflated tyres will result in a slower recorded speed.

 

Even a tiny change in diameter - of say, a few millimetres - will result in an incorrect speed being recorded. This likely error margin is factored in to how car makers calibrate their speedos.

 

So why is my GPS satnav accurate?

GPS satnav systems calculate your speed by using satellites - and assuming the signal is strong, they should be accurate. Some sat navs installed at manufacture are integrated with the car's own measurements to provide a more accurate speed reading.

 

Under UK law - which is based on a European Union standard - speedometers must never underreport a vehicle's speed, while it must never overreport by more than 110% of the actual speed + 6.25mph.

 

So if you’re going 40mph, your speedometer may read up to 50.25mph - but it can never read less than 40mph. In order to stay within the law, carmakers calibrate their speedomters to slightly overreport their vehicles' speeds.

 

This of course means many motorists are travelling slower than they think - which is arguably good news for avoiding accidental speeding tickets and for making our roads safer.

 

You may well find your satnav’s GPS is a more accurate determinant of your speed - although you should stick to your car’s speedometer reading to be on the safe side.