RAC Cars News


Why Your Sat Nav Makes Your Drive Faster

By raccars Published

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Since the sat nav became ubiquitous in British cars, drivers have been noticing a discrepancy between the speed of travel given on the device and that of their car's speedometer. The car usually reports speeds a few mph higher than the sat nav. Given the importance of following the rules of the road and speed limits in particular, which given speed should you rely on?

Speed is usually measured by calculating how long it takes an object to cover a certain distance, however car speedometers don't work like this. They work out how many rotations of the driveshaft or axle or even wheel are being performed, which are calculated to equate to a certain speed. This means that any divergence from the original size of tyre or wheel can throw these calculations off. Even the amount of tread or pressure in a tyre can alter its diameter, albeit in tiny amounts, but over thousands of revolutions that adds up to differences in the speed recorded. The law allows car speedometer manufacturers to build in a certain margin of error to account for this.

A sat nav device measures your car's speed in the more traditional manner, using the time taken to travel a certain distance as recorded by GPS. The size or condition of your car's tyres won't make any difference but the accuracy of the measurement can be affected by the quality of satellite signal or if the car is travelling over a gradient - not all GPS systems can recognise a change in vertical direction. Overall the sat nav is usually more accurate as it is based upon actual driving behaviour.

Given the importance of a speedometer to road law, regulations are in place which, in the UK, for the most part follow the EU standard. A speedometer must never display a speed lower than that being actually travelled, and must not display a figure higher than the actual speed by over 110% plus 6.25mph. What this means is that if your car is actually travelling at 50mph, your speedometer can legally read up to 61.25mph but must not show any lower than 50mph.

To make sure the law is adhered to at all times, manufacturers tend to set up their speedometers to artificially inflate the speed shown a little, building in a margin of error. As sat navs aren't deemed to be used to follow the law in this area, they don't need to jump through these hoops.

In other words, your sat nav may be more exact but cannot be used to justify any speeding. You must rely on the speedometer, accurate or not...

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