Wondered why you're driving at 100km/h and cars are overtaking like you're standing still? It could be their speedometers are more accurate than yours.

Many motorists have been in touch with us about the accuracy of their speedometers. They have cause for their concern.

The rise of in-car GPS units has given drivers another way of measuring their speed. Only one driver who contacted us had a speedometer accurate to within 1km/h of their GPS. Most were reading about 10 percent “slow” (the GPS reading 90km/h while the speedo reads 100km/h). One said the speedometer on his 2009 Subaru Legacy was 18 percent out.

This is no surprise to us. But we’re disappointed that nothing has improved since our speedo accuracy test in 1999. Our tests then showed some speedos were out by only one or two percent – but others were 10 to 15 percent out.

Speedometer standards

All vehicles capable of 50km/h or more and registered in New Zealand after 1951 need a speedometer that’s “in good working order and operates while the vehicle is moving forwards”. But there’s no definition of what constitutes “good working order”.

Australia has standards for speedometers similar to those of the European Union; and most car manufacturers aim to adhere to these standards. The rules say that no speedometer should read lower than the speed you’re travelling – so you should never be going faster than what the speedo reads.

But in terms of your speedo reading higher than your actual speed, a huge margin of error is allowed. For a car, the international standards allow a speedometer to be higher by as much as 14km/h at 100km/h – which means your car is doing 86km/h when your speedo is showing 100km/h. For a motorcycle, 84km/h actual speed is allowed to read as 100km/h on the speedo.

Why so innacurate?

The argument from car companies is that, if the car has low tyre pressures or modified wheels and tyres, there’s a risk the speedo will show a speed that’s lower than the actual speed. So setting the speedo higher makes sure this doesn’t happen. In more litigious countries, setting the speedo higher also prevents car manufacturers from being sued for the cost of its customers’ speeding tickets.

The New Zealand Transport Authority says any speedometer inaccuracy is likely to result in motorists driving slower, not faster. It also says that “there would be little road safety value in mandating a time-consuming and costly check of speedometer accuracy”.

But when it comes to very inaccurate speedos we disagree. Traffic is safest when everyone is moving at the same speed. Someone driving at an indicated 90km/h with a speedo reading that’s 18 percent too high will in fact be travelling at 74km/h. Another vehicle with an accurate speedo could be travelling at 100km/h – so that’s a total speed difference of 26km/h. If you’re travelling too slowly, you’ll get a queue of vehicles and this may tempt other drivers into recklessness.

Also, if you replace a car that has a very inaccurate speedo with one that’s accurate, you’ll start driving faster without realising it. What happens when you arrive at a tricky corner not realising you’re going too fast?

Are GPS speed readouts accurate?

We recently tested in-car navigation systems against a vehicle's speedometer accurately calibrated at 100km/h. On a straight stretch of road, we found the speed-reading of all the units was within 1km/h of our true speed at 100km/h.

However, fluctuations in speed and direction (such as fast acceleration and turning tight corners) cause GPS units to lose speed-accuracy dramatically. Atmospheric and other technical aspects of satellites can also cause errors to GPS accuracy. So don't use your GPS as a fail-safe speedo.

You can do your own speedo check if you have a GPS. Do this on a straight stretch of road, preferably using cruise control to ensure a constant speed. After doing this several times at different speeds, factor the errors into your speedo. Once you’ve mentally adjusted for the margin of error, you can continue to use your speedo as your main speed-readout.

What about the odometer?

Inaccurate speedos especially concern drivers of diesel vehicles. They assume an inaccurate speedo also means an inaccurate odometer, which would result in them paying too much in road-user charges. Fortunately, most odometers are accurate and calibrated differently from the speedo. The international rules say an odometer must read within four percent of the distance travelled.

If you're concerned about your odometer's accuracy, don't test it using the speed readout of your GPS. Instead, do several point-to-point tests using the GPS distance readout over straight roads of 5km or more.

Our view

• It's reasonable to expect speedos to be slightly inaccurate, since tyre pressures and other factors can affect their accuracy. But we think anything over 5 percent out is unacceptable, and more than 10 percent out is ridiculous.
• The Consumer Guarantees Act (CGA) says that products must be free from minor defects and fit for their normal purpose. We think a speedo that’s more than 5 percent out is not fit for purpose and should be rectified by the retailer or manufacturer.
• Some manufacturers such as Ford and Holden can recalibrate their speedos, but others can’t. Instrumentation specialists can recalibrate most speedos for around $400.
• Don't sit in the fast lane, driving at an indicated 100km/h, in the belief that no one should travel faster than you. You could be doing 85km/h. Be considerate and keep left – no matter what your speed.

More from consumer.org.nz

• Car GPS - our March 2010 test
• Car buyers' rights - a free guide to your rights

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