Check out this post. I fixed the MPG problem in my car by pulling the fuses for the OBC. These are E38 fuses, but try pulling the corresponding X5 fuses.

http://www.bimmerboard.com/forums/posts/174341/

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input