Safe Speed Forums

They still won't under read though, so moot point.

From Wiki -

UK
The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.[11]

The Motor Vehicles (Approval) Regulations 2001[12] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
But I've read somewhere else ( possibly Older versions of HC or C & U regs) that a vehicle must have a working speedometer reading speed in excess of 10mph . This is in a post on this site ,although I can't find the particular item .

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lets bring sanity back to speed limits.
Drivers are like donkeys -they respond best to a carrot, not a stick .Road safety experts are like Asses - best kept covered up ,or sat on