With the mods in my sig I get 14 city. Cams and blower.....woohoo! I own 3 vehicles, stang gets the best mileage.

 

With the GT, the heads did change sometime in 08 - not sure if any performance was gained or lost with that change, and I would bet that the stock tune was also tweaked each year. Things were moved around enough within the engine bay for there to be several differences in the Whipple supercharger kits for the different years.

 

I get 28 hwy going 10-15 over the speed limit, so it does pretty good.
My car is stock and it really cruises well at around 80 mph and gets good MPG.

There is something different about the '05-'06 4.0s and the '07-'10 4.0s.
I'm not sure what's different though, I just know that some things that you buy for '05-'06 V6s is specified for those two years.