I have a 07 328xi and I have been having the same problem. My gas mileage isn't as bad as your, its around 16-17 city which I think is prety low. Highway is about 28 which I think should be higher. I would try what the guy said about going on the highway, putting the car on cruise at 65 mph and resetting your mpg.
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Alpine | Carbonstructure | 6-Speed
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