MD has it right on, as usual.

One of his links even has a scan of the BMW TSB that states BMW's position on speedometer accuracy (the speedometer can read up to 10% + 2.4 mph over, but may not read under at all)

10% high reading on the speedometer is typical at highway speeds on a BMW.

If you transition from clear payment to packed snow at highway speeds, you will find even more error, due to tire slip on the snow. That one surprised me, I didn't expect it to be so noticeable.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue