No SUV thus far has garnered truly respectable gas mileage, and that's largely a matter of physics: heavy vehicles that must accelerate with normal traffic simply require more fuel. BMW has mitigated that thirst somewhat, however, particularly in the six-cylinder models. The X5 xDrive35i, for example, rates 16 mpg in town and 23 mpg on the highway for 19 mpg combined. That's not fantastic, but it beats many full-size trucks while seating five in luxury.
The xDrive35d diesel model is the gas mileage hero of the range, however, scoring 19 mpg city and 26 mpg highway for 22 mpg combined. That's competitive with many midsize non-luxury sedans, and very good for an SUV of the X5's girth.
As you'd expect, the V-8-powered xDrive50i model doesn't do as well, rating 14 mpg city and 20 mpg highway for 16 mpg combined. The X5 M is even less green, not that anyone buying a $90,000-plus super-sports SUV will likely worry about a few extra dollars at the pump; it rates 12 mpg in town and 17 mpg on the highway for 14 mpg combined.