Speedos are not allowed to under-read, but are allowed to over-read by up to 10 per cent. In fact, almost all over-read by at least 5 per cent. So 75mph on the average speedometer is between a true 67.5mph and 71.25mph. No one is ever going to get done for 71.25mph so, as you rightly condemn yourself, you are "mistaken". If you have a decent GPS system you can check this for yourself. Many TomTom GPS systems have the facility to show your speed 100 per cent accurately across the surface of the planet, including factory-fitted TomToms. They aren’t as responsive as a speedometer, but they are accurate.