I went on a road trip...a 6 hour drive. I always use 91 octane and this would be no different. The first leg of the trip from Topeka to STL I got 27mpg, pretty good for a V8 car right? Then I filled up at a Mobil station in STL with 93 octane, really this was just to do it, I didnt expect any gains at all, but from STL to Indianapolis I got 30.2mpg which shocked the heck out of me! As I was leaving Indy I filled up with 92 octane which is the highest I could find around my hotel, and from Indy to STL I got 28mpg. Stopped at the same Mobil station in STL and filled up with 93 again, and the car did 30.1mpg back home to Topeka. This is MY experience, and why I just always use premium. I am sure there are variables, but honestly I thought these results were really great considering how my car is equiped. Ford said 26mpg HWY on the new car stcker for this car, so to beat that is impressive to me.
Current mods that COULD effect MPG...
1. O/R H-Pipe
2.
Magnaflow Catback
3.
Eibach Lowering springs( I figure aerodynamics may change and help)
Mods that SHOULD lower MPG
1. Heavier aftermarket Bullitt wheels
2. Heavier BFG tires in front
3. Heavier Goodyears to fit the 17x10.5 wheel in the back
4. Over 100 pounds of luggage in the trunk
Again this is purely my experience, and not based in science at all, just an FYI type thing!