Welcome aboard, Natalya!
In my experience with the 4.0 Liter V-6 Mustang, the largest gains in MPG are from "adjusting the nut behind the steering wheel'. I'm running my tires at 50 PSI (sidewall max is 51 PSI).
Switching to 0W-30 synthetic motor oil from the factory 5W-30 gained about 1/2 MPG. Installing an ASP under-drive crankshaft pulley gained close to 1 MPG - I made it through this past Winter without dropping below 31 MPG.
Try to limit your RPM to 2500 while accelerating (most of the time, if you can
), limit your freeway speed to 65 MPH (if you can).