Don't know where you pulled those numbers from but the way I see it, 34 is over 20% more than 28. Further, my wife gets 23mpg mixed in her 2011 328. In an F30 loaner, she gets 30 mpg over the same routes, same speeds and so on. That is near as makes no difference to 33% more miles per gallon. Put another way, once her e90 has run dry, the F30 would have 112 more miles left to empty. 112 miles
This difference only becomes more significant the higher gas prices go.
Factor in the fact that the F30 extracts no performance penalty whatsoever for all of this economy. It is faster than the car it replaces. A lot faster. Don't forget the vastly upgraded available technology: improved iDrive interface/navi, 8-speed automatic, start/stop system, M adaptive suspension, HUD and so on. For nearly all 3-series buyers, the F30 is the more desirable car.