Why not try driving with a light foot on the throttle for one tank, and see if you notice a difference in the fuel economy? I never got worse than 14 l/100 km on my 2003 3.0, and that was in purely stop and go city driving. Better than that in mixed or highway driving.

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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