My wife's 07 X3 has about 45,000 on it and when it was new it got around 27 MPG on the highway and around 20 in town. Then a few years ago all of a sudden it dropped to maybe 20 on the highway and about 16 in town. I took it in but the dealer said they couldn't find anything wrong and I just didn't have the time or energy to argue with them. It was strange that it was so sudden and such a major change. It runs and drives perfect and with the little she drives it its not worth the effort. I will however watch this thread to see if anyone gets their mileage to improve and see if it looks solvable.
2015 535i M-Sport/ Nutmeg Extended Merino
Cold Weather Package
Driver Assistance/ DA Plus
and a few other toys