Congrats. If you are doing 80% highway driving you must be driving into a stiff north wind if you are getting 15 MPG. You should be getting at least 20 MPG. I get like 19-20 MPG on the highway.

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2008 X5 4.8
Jet Black/Black Nevada/Dark Burl Walnut
Adaptive Drive
Premium Pack
Tech Pack
Climate Pack
Sport Pack
20" Wheel Option
Premium Sound
Sirius Satellite
Rear DVD Entertainment
Multi-Contour Seats
Comfort Access
3rd Row Seat
Heated Front Seats
Running Boards
3M VentureShield Paint Protection Film