Why your speedo is unlikely to be accurate

Many readers will probably be aware their vehicle’s speedometer isn’t accurate and often reads higher than a speedo app or GPS sat-nav system.

But why?

It all comes down to Australian Design Rules (ADRs), which forbid car-makers from under-reporting a vehicle’s speed by any amount and actually require speedos to be anywhere between perfectly accurate or up to 10 per cent plus 4km/h pessimistic.

“ADRs require a speedo tolerance of zero per cent under to 10 per cent above the actual speed, so manufacturers typically set it at about five per cent over,” says University of New South Wales (UNSW) emeritus professor Michael Regan.

“This means the speedo is likely to read 100km/h when, in actual fact, your real speed is 95km/h.”

Again, this probably isn’t much of a surprise to many readers who use third-party apps like Waze and Google Maps via smartphone mirroring in their cars, but now you at least know why the average speedo typically reads a bit under.

But there’s still more to it than that, because the accuracy of a car’s speedo subtly decreases over time as the tyres wear, because the factory speedo calibration is based on a brand-new set of tyres – when its circumference is at its maximum.

“Over time, as the tyres experience normal wear and tear, they get smaller in circumference,” Regan said.

“This changes the accuracy of the reading of the speedometer – again showing a higher speed than the actual speed.

“So if you’ve had tyres on your car for a long time, and the tread on the tyre wears away over time, that means that the wheels are revolving faster than they would be if your car was fitted with brand-new tyres.”

The workaround for disgruntled drivers or those who were previously unaware of this deliberate under-reporting of speed is to fit higher-profile tyres when the time comes for a new set of hoops, such as going from a 215/40 to a 225/40 or from a 265/60 to a 275/60, which are also wider and therefore offer more grip.

The extra couple of millimetres added to the sidewall yields a bigger tyre circumference overall and in turn reins in that factory tolerance, but it should be noted that not all tyres, vehicles and speedos are created equally and so you should do plenty of homework to ensure you stay within the law before making such a move.

Aftermarket or replacement tyres fitted to passenger vehicles must not be more than 15mm larger or smaller than the largest and smallest tyre sizes respectively listed on the placard. Other specifications (load rating, speed rating, etc) listed on the placard are also required to be followed when replacing the tyres that were sold with the car when it was new.

Alternatively, you can just stick with your GPS devices, or determine how much your car’s speedo is out and drive accordingly.

Those loyal to their GPS set-ups should know that while they’re often more accurate than a mechanical speedo, there are still a few caveats to be aware of.

“If you’re driving on a flat, straight road, the GPS is likely to be more accurate than what’s displayed on your speedo,” Regan said.

“However, if you’re going up or down a steep hill, the actual speed (for example, as measured by police mobile radar) will usually be greater than the GPS value but proportional to the steepness of the road you’re travelling on.

“In theory a clever GPS device could account for the road steepness and adjust the displayed speed so it is more accurate.

“However, this is a relatively rare situation and there is no strong justification for navigation devices to make this adjustment.”

Regan said he hoped regulations would one day encourage or even require the fitment of an accurate GPS-based speedometer in most vehicles in the name of accuracy, reliability, safety and convenience.

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