Not sure how many people actually measure their mpg by miles covered/fuel to top up tank method rather than relying on the computer readout. I hazard a guess that those using the computer only are kidding themselves.
My 530d in which I have now done almost 90k miles is always at least 8-10% over reading on computer, previous rides 525d, E240, Audi A6, Omega 3.0, 523, A6 have all given an overread.
For the record the BMW 525d actually gave a "genuine" 42.1 over 80k
My R129 SL500 has given a best of 27 mostly motorway and a worst of 17 mostly around town.