Car speedo vs GPS – which is correct?

If you’ve ever compared your car’s speedometer reading to the speed readings on a GPS device, you may have noticed that the speeds don’t quite match up.

This is most likely because in Australia, speedo readings in vehicles must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 per cent plus 4 km/h.

Speedos must always read ‘safe’, meaning that the vehicle’s true speed must not be higher than the speed indicated by the speedo. In turn, most vehicles’ speedos will read slightly higher than the vehicle’s true speed.

Generally speaking, GPS is very accurate and more so than the speedo, but it doesn’t work all the time, especially in built up areas where there are tall buildings or in areas where there are particular terrain features such as mountainous areas or ridges and cliffs, all of which can interfere with the GPS signal.

There are also safety benefits in relying on your speedo, which will also give you a better chance of avoiding speeding fines.