If you're devoted to keeping it Mustang, look up
Ultarc and what he's done with his 3.7 kind of tips the paradigm of sports car ≠ fuel economy. Those are the kinds of numbers people are usually pretty happy to get with their Corolla, and even then they don't have the kind of power on tap Ultarc has.
Whether that can be achieved with a newer 'Stang and its engine choices is a different question. I would be VERY curious to have someone come here with a 2.3 EcoBoost and hypermile the nuts off it, and tell us how he did.