Just repeating what I posted on another thread but UK law based on EU regs requires car manufacturers to provide speedometer accuracy in the range of 0 to 10% plus 6.25 mph over read in the true speed range 25 to 70 mph. It cannot read below the actual road speed therefore and the over read is limited to the formula is V/10 + 6.25 mph where V is the true speed.
This means a vehicle traveling at a true 40 mph could have a speedometer indicating 50.25 mph and still be legal
In reality most seem to read around 5% over I've found. Having said that my 55 year old Land Rover is spot on courtesy of 7.50 tyres
