Even though our fuel costs more than in the US, there isn't a big financial prize in trying to improve mileage. But I consider it a personal challenge to drive smoothly and efficiently, especially on a long trip where I have a quantifiable indicator of that efficiency, and a so I always see what I can get on a tankful. The X3 has a 67 litre tank instead of the 92 litre tank on the E53. The engine is a 3.0, 265 hp. The vehicle is lighter than an X5 but has the same drivetrain. I try for 800 km (500 miles) per tank on the highway, and can nearly always get 760+. We just came back 1000 km today in snow, through the Rockies, temps ranging to -16. 780 km on the first tank. Should have used midgrade since there isn't a reason for using the premium in the lower ambients, but I used premium to minimize the ethanol contamination.

My quick calculations show that as 8.4 litre/100 km, 34 mpg Imperial, 28 mpg US. Not great driving conditions, and some traffic, heavily loaded with 3 people and luggage, so that didn't seem bad.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue