Legally, speedometers can read upto 10% above true road speed. They cannot read below true road speed.
The source is on the internet if you can be bothered to waive through the bureaucratic language of a goverment bill.
It's this legal requirement that created the myth/legend of police officers allowing drivers to speed by upto '10% + 2mph' before issung a ticket.
Really? I thought it was the official ACPO or whatever their called guidelines stating as such
The ACPO guidelines set out the minimum figures at which a Fixed Penalty Notice and a Court Summons would normally be considered appropriate for a speeding offence, if there are no other aggravating circumstances. The minimum figure for a FPN is the posted limit + 10% + 2 mph. These figures are shown in the table below:
The ACPO Speed Enforcement Guidelines
Speed Limit (mph) Fixed Penalty Summons
20 25 35
30 35 50
40 46 66
50 57 76
60 68 86
70 79 96
The full details can be found on the ACPO website.
Bear in mind that exceeding a speed limit by any amount whatsoever is an absolute offence, and a police officer is fully entitled to charge a driver for doing, say, 31 in a 30 limit outside a school at closing time, or in a busy High Street.