SPEED

FAQ – Is a SAT NAV more accurate than your speedometer?

 

ANSWER

The law for car and motorcycle speedometers in the UK


The UK law is based on the EU standard, with some minor changes.  

A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph.

 

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, vehicle 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 or bike’s speed is measured, it does not need to incorporate any fudge factoring.  Neither can it be used as a defence against a prosecution for speeding.





Margin of error in a car or motorcycle speedometer

Car and motorbike speedos usually work by measuring rotation of the car’s driveshaft, axle or wheel.  They then use some basic maths to determine how fast you are travelling.


The speedometer is designed to work with the standard specified wheels and tyres.

If the wheels or tyres are non standard e.g larger or smaller wheels with low profile tyres or off road tyres they can significantly change the speedometer reading down or up.

If the diameter decreases (eg – worn tyres, less air in the tyres, a different brand of tyre with slightly different dimensions, more load on the vehicle weighing it down and compressing the tyres),  also as a motorcycle leans to travel through corners the contact patch moves to the side of the tyre which is a smaller rolling diameter.  

In these circumstances the vehicle will be travelling a shorter distance for each revolution of the wheel, and you will be going slower than the indicated speedometer speed.

 

The diameter will increase if you put new tyres on the vehicle (more tread) or increase the tyre pressure.  This means that, for each revolution of the wheel, the car is travelling further, meaning your speed is greater.

The differences in wheel diameter resulting from the above circumstances could be tiny (maybe a few millimetres), but at 30mph your 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 vehicle speedos.




How a satnav speedometer works


Is the speed on your satnav more accurate than your speedometer?  

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 tyres.  Many satnavs are unable to account for changes in vertical direction, so may be less accurate if you are travelling up or down a steep hill.  They are also inherently more accurate at higher speeds in a straight line over level road, as a larger distance over time reduces rounding errors. but a satnav will usually be much closer to a vehicle’s true speed than the speedometer.  However not always; remember that the sharper the corners are on your route, the blind spots while passing through trees, tunnels, deep cuttings etc can all potentially cause the GPS system to record an inexact distance travelled and show a lower speed than you are travelling.


Conclusions


A satnav will usually be closer to a vehicle’s true speed than the speedometer.

Can you rely on it to always give a more accurate speed.  NO.


Your satnav is not the designated device by which a car or bike’s speed is measured, neither can it be used as a defence against a prosecution for speeding.


It is possible that you may travel in excess of the speed limit as a result of following the reading on your Sat Nav and you risk a prosecution.