Mika is quite correct with the 10% error margin and most manufacturers do stick towards this top end error margin as they would be seriously penalised by the authorities if it was ever shown that the speedo read below the correct speed.....Regarding Sat Nav accuracy do be aware that the Garmins and tom toms etc will achieve upto 1% accuracy ( other 'lesser 'sets may be far worse ) in a straight line and on leval roads at a steady and settled speed..Also our GPSs refreash every second or so,....this is the reason why the display jumps up under acceleration/decellaration. ,so it is hard to compare one reading with another as errors will occur in the time it takes for our eye to focu and read both screens .. In the real world its best to think that your Sat nav is possably within within 5% of the speed shown on a radar/laser gun used by the authorities !! . ...... If you Google 'speedo healer v4'.. etc..you will find a gizmo that may be of interest.... I don't have one and know no more about it than what you will find on the web sites so cant vouch for their claims..