I'm curious how tall 6th gear is. With that engine, just feathering the throttle will probably do the trick for typical highway speeds.
Here's some CAFE stuff for anyone interested:
C A F E Overview
Interesting that production volume is part of the calculation. I guess that since fewer Mustangs are sold than Fusions or Focuses, a slightly lower FE won't be too bad a penalty.
I also found it interesting that failure to reach CAFE is penalized by tenths of a mpg. That's pretty precise, since the EPA only posts figures rounded to whole numbers.