I just took my second "road trip" with the 4.8 and noticed that if I keep it around 80 mph I couldn't get any better than 17 point something mpg. In between 70 and 80 mph it was 18 point something. At 70 mph or less it would get about 19mpg or even a little higher. The trip was about 1100 miles so I went through a few tanks of gas.
Around town it still looks to be 13-14 mpg but we live in a fairly congested area and traffic sucks most of the time.

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2007 X5 4.8i Space Gray
2001 X5 3.0 Titanium Silver