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What law says speedos are not to under-read?

Is there a law which says speedos are not to under-read?

Asked on 5 May 2012 by D.M., via email

Answered by Honest John
Regulation 35 Road Vehicles (Construction and Use) Regulations 1986 specifies that the car must have a working speedometer. EC Regulation (Community Directive) 97/39 or ECE Reg 39 refers to 75/443/EEC that specifies the tolerances. The indicated speed must never be less than the true speed (it must read exact or high) and between 25mph and 75mph the error must not exceed 10 per cent plus 2.5mph high.
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