How accurate is my speedometer?

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? Are car speedometers accurate?  Or is your GPS displaying the correct speed?  

In nearly all such cases, the car speedometer accuracy 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?

How does a car speedometer work?

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.

How do my tyres impact the accuracy of my car speedometer?

If the tyres or wheels are changed, this will impact the accuracy of the speedometer reading.

A larger diameter tyre 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.

With this in mind, if you’ve changed your tyres on your car recently, you may be wondering if you need to do anything to ensure the car speedometer’s accuracy is not affected.

Speedometer calibration: Does my speedometer need calibrating?

The likely error margin of the speedometer’s accuracy is factored in to how car makers calibrate their speedometers. Car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount.

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.

UK speedometer regulations

Under UK speedometer regulations, all vehicles registered in the UK since 1977 have been required to have a speedometer capable of displaying speeds in kilometres per hour (km∕h) as well as miles per hour (mph).

Foreign registered vehicles are generally not required to have speedometers capable of displaying miles per hour. 

Speedometer accuracy: What does the UK law state on how accurate speedometers must be?

Under UK law - which is based on a European Union standard -  UK speedometer regulations also outline how accurate speedometers must be.

The regulation states that 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 speedometers to slightly overreport their vehicles' speeds.

Car manufacturers speedometer calibration UK

In order to stay within the law, and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, carmakers calibrate their speedometers to slightly overreport their vehicles' speeds.

So, in answer to are speedometers accurate, it is clear that the calibration of speedometers by car manufacturers 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 sat nav’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.

Technological developments also look set to change the way we drive, and the speeds we can travel.

Mandatory speed limiters

After several years in the pipeline, Intelligent Speed Assist (ISA) technology is set to become mandatory on all new cars in the UK from July of this year.

Some cars are already fitted speed limiters installed. These existing limiters can usually be overridden simply by pressing down hard on the accelerator. However, the capability to turn off speed limiters in your vehicle may change in the coming months or years. Find out more about the proposed introduction of mandatory speed limiters.