I heard from one of the German engineers that speedometers are usually set up to show about approx. till 10% over the actual speed. Therefore, the faster you go, the more speedometer is lying
The current UK law is based on the EU standard with some minor changes…
A speedometer must never read less than the actual speed or show more than 110% of actual speed + 6.25mph. So, if your true speed is 40mph, your speedometer can legally read up to 50.25mph, but never less than 40mph. In other words, if it reads 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be traveling at 40mph.
Car manufacturers will normally calibrate their speedometers to read ‘high’ by a certain amount in order to adhere to this law and ensure they never show less than true speed.