Speedometers must not under read by law. They can read accurately or over read, but not under read. IIRC under the old Con & Use Regs it was a straight 10%, now it is under the EEC directives I believe it can read up to 110% of true speed + 6.25mph, so 39.25mph at a true 30, 50.25mph at a true 40, etc, etc. The likelihood of one reading accurately is slim, as the manufacturer runs the risk of it under reading if they are fractionally out, and they also need to build in some degree of tolerance for the change in rolling diameter of the tyre through wear. Surprised at the Cheshire Constabulary one as the ACPO guidelines were 10% + 2 before prosecution, but they were only guidelines.
I've always got a GPS speed running in whatever I driving/riding (I.e. Waze) and the speed is always lower than the speed shown on the vehicle speedo so I know I have some contingency to play with.
