How fast am I really going? The accuracy of speedos

This week at RACQ we were asked a tricky question: which is more accurate – your speedo or your sat nav?

Answer: Vehicle speedometers must comply to Australian Design Rules which stipulate they must not have an error greater than 10 percent plus 4km/h on the ‘safe’ side. RACQ has found speedos typically have around five percent error on the ‘safe’ side.

That all sounds technical but essentially it means that if you’re travelling at 100 km/h, the speedo must read between 100km/h and 114km/h. Due to the ‘safe’ requirements, the effect is that motorists may find they’re driving at a true speed that’s slightly less than what’s showing on the vehicle’s speedo.

Satellite navigation devices don’t have to comply to these design rules, so in essence, while they may be more accurate, the driver has no way of knowing what error tolerance is in the device. The speedo that comes with your car is the one that has to go through the checks and regulations, so it’s the one you should rely on.

Here’s another thing to keep in mind: a speedometer reading can be influenced by your wheels and tyres. If you’ve retrofitted hot new rims, make sure you get your speedometer accuracy re-tested. You can give RACQ’s Motoring Advice line a call and they can advise where you can get this done.

Finally, if you’re prone to using cruise control, remember it’s not a ‘set and forget’ item. It’s up to the driver to regularly check the speedo once cruise control is activated and ensure their vehicle stays within the posted limit.