And btw I checked out several BMW sites from the UK, Aus, Poland, etc. While the 320i and the 328i are rated the same for fuel economy, they are not rated the same elsewhere. The city economy is significantly higher for the 320i than the 328i.
Using the American CAFE tracks for city and highway they may score quite similarly, but the other tests give the 320i the edge in economy. In fact, if you look up the the 2012 320i models that have been reviewed the observed mileage is noticeably better than that of 2012 328i's tested.
Check out even the EU site for BMW and you'll see that the 320i has a higher average fuel economy over the 328i even as the 328i has a similar extra-urban rating as the 320i.
All that being said, for all intents and purposes, there will be a noticeable difference in fuel economy between these two vehicles (especially for people who drive in the city).