Well - according to German law the speedometer can only have a tolerance to the high side. It can never show a speed lower than the one you are driving.
The formula is
actual speed < speed shown by speedometer < actual speed + 10 % plus 4 km/h
§ 57 Par. 2 StVO
Anlage zu § 57 Par. 2 StVO
I assume there are similar laws in other countries.
PS The Prius is a big fat liar on speed and fuel economy.
The roadside radar almost always shows +1 mph, the few times when it doesn't show +1 it's zero.