Welcome aboard, fellow Mustang owner! I know from my own experience that it's possible to average over 30 MPG commuting with a 4 liter V-6 S-197 (2005-2009) Mustang. The mods that made the major differences in fuel economy were "adjusting the nut behind the steering wheel" and adjusting my commute route to eliminate driving @ 75 MPH (or driving @ 65 MPH in the 75 MPH speed limit areas). I live in northern Arizona - rarely need A/C unless I have a passenger.
Best of luck with your project.