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Can you trust your car’s speedometer?

10:08 13 March 2016

Trust your cars speedometer and it will keep you within the law

Trust your cars speedometer and it will keep you within the law

PA

Is your 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 it’s important that the speed you see is accurate.

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 speedo is not going to start telling you are driving at the legal limit when, in fact, you are going faster.

Don’t panic

Cars made in modern times are built with speedometers 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 you smartphone.

Don’t panic about this. For one thing, if the speed on the sat-nav or phone is lower than the one on the car’s dash, you are safely within legal limits. Sat-navs and phones use GPS tracking to calculate speed based on time and distance – great for military aircraft and ships but not so good for cars on normal roads as it doesn’t account for undulations and curves in the road.

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

Margins

So, all cars’ speedo’s 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 tyres wear down. However, as the tyres wear, the speedo is only likely to over-read very slightly more, not less, so it won’t risk you speeding when you think you are within the law.

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 your tyre supplier because 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.

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