Is your car lying? The truth about speedometers

Published: Saturday 5th March 2016 by The News Editor

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It’s one of the great conundrums of the modern world: is my car’s speedometer telling the truth? None of us wants to end up with a speeding ticket when we think we’ve been travelling at the legal permissible speed, so is a speedometer accurate and trustworthy?

The simple answer is yes, you can trust your car’s speed reading. While some much older cars that are now very much classics might have speedos that tell fibs, any car built from the 1980s onwards will have a speedo that is reasonably accurate. Even allowing for wear, tear and age, a speedometer is not going to start telling you you are driving at the legal limit when, in fact, you are going faster.

Cars made in modern times are built with speedos that deliberately over-read by a small amount. This is usually not obvious at lower speeds in town or even on 40mph stretches of road. Head on to faster roads, such as a 60mph limit or the motorway and you may notice some discrepancy between the reading on your car’s speedo and that on a satellite navigation system or an app on your smartphone.

Don’t panic about this. For one thing, if the speed on the satnav or phone is lower than the one on the car’s dash, you are safely within limits. Another point to bear in mind is satnavs and phones use GPS tracking to calculate speed based on time and distance. While this is great for military aircraft and ships, it’s not quite so great for cars on normal roads, as it doesn’t account for undulations and curves.

The main reason cars’ speedos over-read is because car companies do not want to face any legal action for a driver going too quickly. This could be legal action from a driver who feels a false reading has caused them to be prosecuted for speeding or it could be a criminal prosecution if someone was injured as a result of excessive speed.

So, all cars’ speedos over-read, but this has to be within a 10% margin, as it could be just as dangerous for a car to be travelling too slowly on a road and cause hold-ups and frustration for other drivers. The speedo must never under-read, so if it says 60mph, it cannot be any more than that speed.

A worry for some drivers is the speedo might become incorrect as the cars’ tyres wear down. However, as the tyres wear, the speedo is only likely to over-read very slightly more, not less.

What can catch you out is if you change to a set of larger wheels and tyres. If the rolling circumference of the new tyres is larger than the original ones, the speedo can begin to under-read and put you at risk of speeding without knowing it. Check with you tyre supplier or you may have to get your car’s speedo recalibrated to allow for this. Otherwise, trust your car’s speedometer and it will keep you within the law.

Copyright Press Association 2016

Published: Saturday 5th March 2016 by The News Editor

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