Every single car I have ever owned has undervalued the speed. The speedometer is designed that way. It's an insurance buffer.
Imagine if every time you changed tyres or added aftermarket wheels you also had to recalibrate the speedometer. Why do you think the police vehicles need to have their speedometers recalibrated regularily?
Imagine if BMW made a speedometer that might, in certain circumstances (with, say, aftermarket wheels), overvalue the speed. Then imagine people were getting ticketed. BMW would be able to hear the class action calls all the way across the Atlantic!
That said, my speedometer is pretty accurate, according to one of those speed displays they have set up on one of the roads here in Stuttgart. When the needle is pointing at 50km/h, the sign reads between 47 and 49. But NEVER 50.
Also and out of interest, I believe that the speed display that is accessible in the hidden OBC menu is accurate. Or maybe not...
Finally, the speed isn't read from the transmission as one poster mentioned. I actually found that it is read from the rear wheels. About a month ago the street into my driveway was completely iced over. Coupled with the fact that it is on an incline and it's very difficult to get momentum out of the gate, I had to turn DSC completely off and allow the car to spin itself forward. While the wheels were spinning, the speedo was going up and down like a yoyo.
This is first time ever that I have heard someone complain that their speedometer shows a higher speed than they are actually travelling. It boggles the mind.
