Donut
New member
Pretty sure that this question has been asked before, so here is what a quick google search brings up:
The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.
To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount. As your satnav is not the designated device by which a car’s speed is measured, it does not need to incorporate any fudge factoring.
