Failed driving test as sat nav speed incorrect.

vikingaero

If the sat nav shows you were travelling 4mph faster than the speedo then you were speeding. Generally at constant speeds the sat nav is more accurate than the speedo. If the sat nav say 34mph then your speedo must have shown 36-37mph.

As an example, get someone to drive the car at a speedo constant 60mph on a straight flattish dual carriageway. Your sat nav will probably show a truer speed of 56-58mph.