...my speedometer indicates a number somewhat higher than the speed limit. This differs per car but in my case the speedometer is off by 3-6 km/h (the difference increases with speed) according to the average indicated by several SatNavs I tried. I'm assuming these are quite accurate, because they can calculate an average over a somewhat longer distance; I also live in a very flat country.
For example, if my speedometer reads 125 km/h I'm actually doing 120 km/h (that's 75 MPH for the metric-impaired).
The fun thing is that the car system is actively lying about your speed (because the manufacturer wants to stay on the safe side); I have a ScanGauge that indicates a lower (correct) speed as reported by the board computer, which I assume is also the number used to keep track of distance covered. So it seems that the speedometer logic purposely increases this number by some small factor.