Don't do it!!!! If you are wanting to trade only for a 25% increase in gas mileage. You will never see the difference. I had an 01 540 and the fuel economy was no better than your X5. The reason is that those numbers only apply to conservative drivers. Guess what???......there are no conservative 540 drivers. What I mean by conservative drivers is (easy on the gas pedal). Wether it was a quick trip to the store or long road trip on vacation, I always found it hard to not be haulin azz which meant my fuel economy was but I didnt care because it was a blast to drive. Just my .02, GL on your decision.