With the new school year recently started here there are quite a few radar speed devices showing the car's speed. I find that my speedometer over reports my speed by 4 - 8 km/hr. As an example the radar says I'm going 44 km/hr but my speedometer says I'm going 50 km/hr. I've calibrated the radar with my 928GTS and it is spot on, i.e., radar say 50 km/h and my digital readout is 50 km/h.
Now I do have after market 18" rims with Micheline Pilot Sport A/S 3+ with a diameter of 25.7". I'm not sure if there is a difference in the diameter versus the stock 16 rims.
I am assuming that the speedometer measures the driveshaft rpm and then via some cables and gears are used to translate that rpm to km/h. If so perhaps I've got the wrong diameter/number of teeth in the gear that results in an overstating of the speed by 4 - km/hr. I would also assume that the mileage on the counter too is over reporting.