Interestingly it was a road side speed sign that got me thinking about the accuracy of the speedo. The Car was registering 3km/h quicker than the sign too in my case.
My wife's acura is reading 5% more than the speedo/odometer. There was a class action law suit against honda/acura which provided 5% additional warranty because of the odometer reading... I'll post a copy for reference when I find it. EDIT: ya, here is the link:
http://www.canada-odo-action.ca/
hmm, this brings up another issue. If the BMW over reports speed/mileage the warranty should be adjusted accordingly.