Did you get all the maintenance up-to-dated? New spark plugs and O2 sensors (pre-cat) would improve mpg.
There might be some addictives change in gasoline in winter time too.
Also, look like you're running 20" wheels on it, in your avatar. That certainly not help. I used to avg about 15-16mpg on the old 3.0 with 90% city driving.

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00 E39 DINAN S1 //M5 (82k miles)
06 E53 DINAN 4.8iS, born 2/18/2006 (126k miles) http://www.xoutpost.com/bmw-sav-foru...s-my-dslr.html
90 Straman Z32 TwinTurbo Convertible, (1 of 44 ever made) 5 speed, 444rwhp/451rwtq
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