I thinks its part of the BMW marketing, the result is overstated mpg and the fact the BMW feels so solid at a high speed.

My X was off by about 3 MPH according to the Garmin.

fair assumption but 2.4mph at +100mph isn't going to make a unstable vehicle stable, nor would it be enough to phase a x5.

That's funny. I pass by signs that display your speed (no cameras) and it's always a couple mph less than what my speedometer says.

The maximum spec is 10% plus 2.4, so that would be a total error of 12.4 mph at 100. Assuming a bell curve distribution, most speedometers would read about 6 mph, or 10 km/hr over at that speed, statistically.

I read 7% off around 80MPH.