Some vehicles have to have a speedometer that is within 2%, calibrated before use when new, tested every 2 years, and recalibrated every 6 years. The speedos on these vehicles read the same as my ancient tomtom one new edition.The speedo is the legal requirement not the sat nav. Before 2001 speedos had to be within + or - 10% at 30 MPH. That is why all speed cameras are set at +10% +2 MPH. As from 2001 speedos cannot indicate slower than the actual road speed. But they can indicate 10% faster than actual road speed. So at any indicated speed the actual road speed can be lower than indicated but never higher. Tyre deflection with load, tyre wear reduce the rolling radius of the wheel so that is the reason for the allowance below indicated. On a vehicle made in or after 2001 fitting larger wheels and tyres that increase the rolling radius makes the vehicle illegal to drive on roads within Europe, because that will cause the speedo to indicate a slower speed than actual road speed..