Why does my speedo read higher than road side speed monitors?


We sometimes receive queries from AA Members questioning why their vehicle’s speedometer reads higher than their GPS or digital speed monitors on the side of roads. Does this mean that it’s broken or unreliable?

First and foremost, speedometers in most vehicles are designed to overestimate the speed of travel. International law has long required modern cars to overstate true speed. The applicable standard for many 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 a true speed of 90km/h, the speedo must read no less than 90km/h and no more than 103km/h.

This then disqualifies any excuse given to a police officer pulling you over that goes along the lines of “my speedo said I was under”. If the officer says you were doing 105km/h, in all likelihood your speedo would have indicated that you were travelling significantly faster.

During the Christmas/New Year period, a speed tolerance of 4km/h is typically enforced on New Zealand roads to help counter the increased volume of traffic, and prevent the risk of accidents taking place. More and more frequently you hear from some 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 small effect on the odometer reading but it’s the responsibility of the vehicle’s owner to maintain. Tyre pressure can also have an effect on speedometer reading, under-inflated tyres can result in the speedometer over-estimating the true speed.

The standards for car manufacturers, and the effect of wheels and tyres or tyre pressures all mean that each car’s speedometer reading will be different, so while you may think another vehicle is travelling slower than the speed limit, that may not actually be the case.

Speed most certainly contributes to accidents on our roads and the higher the speed the greater the risk of dying in a crash. In 2016, driving too fast for the conditions was a factor in 28% of fatal crashes and 17% of injury crashes (speed limits not specified). Of course driver error and poor handling of vehicles is a contributing factor to the road toll and we all have a responsibility to drive safely this summer but the faster you’re travelling, the less time you have to react in response to hazards and avoiding dangerous situations. And with tourism numbers booming leading to more motorists who are unfamiliar with our roads, 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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