Biggest impact on fuel mileage is the driver and driving habits.

2nd is probably ethanol content of the specific fuel (the actual content, not what they claim it could be on the dispensing pump)

Winter mileage drops are often associated with more idling time. Also, check tire pressures since they will drop with the cold ambient temperature.

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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