I'm surprised that the diesel is only seeing around 21MPG. I have a 3.0 gas X5 and I'm averaging 21mpg on my commute and that includes crossing the Tappan Zee 2x a day. 120 miles roundtrip, GSP>Tap>Merrit. And I agree with the other poster, the engine is getting looser as it gets older, I have ~ 14k on the clock now and the MPG's are getting better.

__________________
You have your way. I have my way. As for the right way, the correct way, and the only way, it does not exist.