Car Care

Why does my speedo overestimate and how do I fare with a 4km/h speed tolerance this summer?

We sometimes receive queries from Members questioning why their vehicle’s odometer reads higher than their GPS or digital speed monitors on the side of roads. Does this mean that it’s broken or unreliable? Less frequently, we hear from people who are asking whether this means that the speeding ticket they’ve been issued can be challenged because the reading was incorrect.

The most important thing to bear in mind, first and foremost, is that speedometers in most vehicles are designed to overestimate the speed of travel.

AA Motoring Affairs expert Mark Stockdale explains.

“International law has long required modern cars to overstate true speed. The applicable standard for most vehicles sold in NZ is a European standard that specifies that speedometers 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 of more than 10 percent plus 4 km/h.”

What does this mean in real driving terms?

Well, another way to look at it is, at an indicated speed of 100km/h, the vehicle's true speed must be between 87.3 km/h and 100km/h.

This in turn disqualifies any excuse given to an officer pulling you over that goes along the lines of “my speedo said I was under”. If you’re pulled over for going 104km/h, in all likelihood your speedo would have indicated that you were travelling significantly faster.

During December and January, a speed tolerance of 4km/h is enforced on New Zealand roads, to help counter the increased volume of traffic on our roads during the holiday season, and subsequently prevent the risk of accidents taking place.

More and more frequently you hear from a number of people who respond negatively to the tolerance saying that it’s not speed which causes accidents, and that the fault is with people behind the wheel.

The tolerance is not changing the speed limit and it’s not reducing it. It just means that if you choose to exceed the limit, you’re more likely to be pulled over so only those who don’t adhere to it in the first place are going to be at greater risk of a penalty. That 4km/h allowance allows a small margin for speed creep and travelling downhill, or those running different wheel and tyre combinations which can, in some instances, have a negligible effect on the odometer reading but are the responsibility of the vehicle’s owner to maintain. Tyre pressure, too, can have an effect on speedometer reading, but under-inflated tyres also result in the speedometer over-estimating the true speed.

The truth, and what we tell anyone who calls us to complain about a ticket, is that if you’re exceeding the speed limit by even 1km/h, you’re at risk of receiving an infringement. It’s safer just to stick to the indicated speed on your speedometer to ensure you remain within the speed limit. While GPS is accurate, we don’t recommend using it to set speed as it provides no margin of error for speed creep or when travelling downhill.

Speed most certainly contributes to accidents on our roads and increases the risk of those accidents resulting in fatalities. In 2015, driving too fast for the conditions was a factor in 32% of fatal crashes and 18% of injury crashes. Of course driver error and poor handling of vehicles is a contributing factor to the road toll. We all have a responsibility to drive safely this summer but the faster you’re travelling, the slower you’ll be to responding to hazards and avoiding dangerous situations. And with tourism numbers booming, leading to more motorists behind the wheel who are unfamiliar with our roads and driving code, it only means there’s even greater reason to adhere to what your speedometer is telling you.

Stay safe if you’re driving this summer and follow your speedo.

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