Something to consider is whether the oxygen sensor needs replacement depending on your odometer mileage. Might not necessarily trip the check engine light but could make mileage suffer. Not sure if emissions results will give an indication of this.
Also I agree with Johnnie Walker, when I had my 323ci the frequent oil changes seemed to help mileage quite a bit.
And if you have steptronic - take it easy in sports mode - might be fun but it burns gas like nothing else.
Be prepared some people might say you can't be preoccupied with mileage when you own a BMW. Maybe buy a few shares in an oil co, in the long run you'll come out ahead.
'07 328i Coupe
Stock for now
Future plans: AA Tune, intake and brake package.