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Which is more accurate - my satnav or my speedometer?

As my car's indicated speed differs by over 3mph on its speedo from what is shown on my satnav (which means there is at least a 10 per cent difference at 30mph), do you know which is more accurate? Also, are we allowed to know what tolerances various police speed cameras systems are manufactured to, as the accumulative error differences could be well in excess of 10 per cent?

Asked on 5 May 2012 by BC, Abergele

Answered by Honest John
On a straight road on a level surface with no obstructions between the satnav and at least three satellites the satnav will be 100 per cent accurate. In other situations it computes your speed so its accuracy can be suspect. ECE Reg 39 stipulates speedometer graduations and refers to 75/443/EEC which specifies that the indicated speed must never be less than the true speed, and between 25mph and 75mph the error must not exceed 10 per cent + 2.5mph.
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