Car speedometers are not allowed to 'under-read' – they can't tell you you're going more slowly than you really are – but they are allowed to over-read by up to 10 per cent plus 6.25mph. So they could read 50.25mph at 40mph.

Are Speedos in cars accurate?

"The accuracy of the speedometer in most vehicles, including Volkswagens, is generally within a few percentage points of actual speed," Tetzlaff says. "Odometer readings are designed to be quite accurate." ... "Overinflation or oversize tires slow down the speedometer."

Are car speedometers calibrated?

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.