Ours is a 2007, with the 3.0si engine (updated one generation from yours).

We get 12 litres/100 km in the city, stop and go, which is 23.6 mpg (Imperial) or 19.7 mpg (US gallons)

We can get up to 8 litres/100 km on the highway, which is 35.5 mpg (Imperial) or 29.6 mpg (US gallons) but you have to be taking it very easy to achieve that.

32 psi, manual transmission, no extra weight in the car. Adding our roof rack reduces the mileage as well.

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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