Are you relying on the on-board fuel consumption computer, or are you filling the tank and calculating it from gallons and miles driven, preferably over several tanks?

Turning off the air conditioning will have a minimal impact, and if you open the windows to get fresh air it may be worse.

If you are not seeing a difference between city and highway it is most likely your driving style.

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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