I can come at this from a 535d perspective. I regularly get 500+ miles from a tank. The tank holds 70 litres, but I've never managed to put in more than 63! Anyway, the OBC for ages was telling me that I've been averaging 33.2mpg, dropping to 32.8 occasionally.
If I never get less than 500 miles from a tank, and have never put more than 63 litres in, then it stands to reason that I should be getting about 33.3 as an average! Damn those inaccurate OBC's. LOL
Having said that, if you don't reset the OBC regularly it'll take ages for the more realistic mpg figure to appear. Mine was showing 19mpg for ages following a dealer visit. it was clearly doing almost twice that, but the OBC hadn't caught up.
Oh, and in response to the first question, the engine will loosen up nicely at about 10k miles and you'll only then get a reasonably clear indication of mpg.
Cheers
Matt