I've gotten 23mpg on longer trips with a fully loaded X5. By fully loaded I mean 4 adults with luggage. I didn't use the trip computer either. I calculated the mileage by dividing the miles by the gallons used. I checked it over long distances and full tanks. This was with the 3.0 of course. I also got over 20mpg with the 4.8 on my last long trip. The key to this is keeping the speed to around 70mph or less. I'd like to know what it would be if I kept it at 55, but you get run over at 55 mph on most highways. In fact, it's hard to kept either X5 under 80 unless you really concentrate or set the cruise control.

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2007 X5 4.8i Space Gray
2001 X5 3.0 Titanium Silver