Safe Speed Forums

As I understand it, vehicles have to be fitted with speedometers that are capable of measuring speed to within an accuracy of 10%. In other words, a car travelling at 30 mph, can have a speedometer reading anywhere between 27 and 33 mph. A car travelling at 33 mph could therefore have a speedometer reading of 30 mph. The speed measurement device is claimed to be accurate to within 1 mph. That means that a speed device reading of 34 mph could be recorded for a vehicle where the speedometer is displaying 30 mph. Hence a vehicle travelling at 30 mph should only be prosecuted for travelling at 35 mph and above, because their speedometer must then be reading above 30 mph. At 40mph this becomes 46mph, etc. i.e. 10% + 2mph. Any conviction at a threshold below this could legitimately be challenged in court, as the driver could easily claim that he/she believed that they were travelling at or below the speed limit, because that's what the speedometer told them, and the device is within the range of acceptable tolerance. Setting the threshold at 10% + 2 mph removes this defence. Hence the reason for the ACPO guidelines. Partnerships ignore this at their peril.

I have just heard from a third party that the partnership operating in my area are starting to adopt this as policy as well, starting from just 31 mph. That means a driver driving what he/she believes to be within the limit could be disqualified from driving within a very short period of time, if they are unable to prove their case in court.

I will be writing to them for clarification.

No. The tolerance is 10% for over-read, but zero for under-read. So if the car is doing a genuine 30mph it's perfectly OK for the speedo to read from 30 to 33, but illegal for a speedo to read less than 30.

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"Politicians are the same the world over... We build bridges where there aren't any rivers." - Nikita Kruschev

That standard only applies at the moment of sale as a new vehicle. There is no accuracy specification or test applied at any other stage in a vehicle's life.

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Paul Smith
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The Safe Speed campaign demands a return to intelligent road safety

Thanks for the clarification. I stand corrected. Is there anywhere this is specified in law then? Or is it a British or European standard?
Paul's correct though as well. It's perfectly feasible for any instrument, and especially a speedometer, to drift by 10%, or even more, over a car's lifetime.

That's correct, but I'm sure this cannot be used as defence in law (even though there is no requirement for it to be checked).

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign