You are at 18 litres/100 km.

On my 3.0, I got 14 l/100 km around town and on short trips, 10.5 l/100 km on the highway when really trying.

18 l/100 km doesn't seem so bad for a 4.6

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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