Here is an article about that very question :
5 Ways Ford Made the 2013 Fusion Sleeker - TheStreet
QUOTE :
The work is expected to result in a 10% reduction in aerodynamic drag, which would lead to a 2.5% fuel efficiency improvement in highway mileage. Highway mileage for the standard 2012 Fusion is 33 miles per gallon, or 36 miles per gallon for the hybrid version. Of the projected drag reduction improvement in the 2013 model, 41% would result from the improved underbody shields, 36% from styling changes and 23% from active grille shutters.
Note that there are 3 pages to the article, so look for the link at the bottom.