Derf00

Let's see, where to start?

As stated the 4.0L is not known to be fuel efficient by any standard and comparing to other cars is just setting yourself up for dissappointment.
Auto EPA for Mustang
City is 16, Highway is 24, Mixed is 19
Manual EPA for Mustang
City is 17, Highway is 25, Mixed is 20

First off, never buy mods on the expectation to get better MPG. Performance improvements and MPG improvements are almost always exclusionary meaning, you can't have cake and eat it too.

Secondly, Replace fuel filter, air filter (or clean it if it's reuseable), change the plugs and coil wires (if it isn't COP).

Third, calibrate your foot as in don't be heavy on the gas, always shift UP when at or around 1500 to 2000 rpm and when cruising and always use the gear that keeps your RPM the lowest without lugging the motor.

Fourth what rear gear is in the car? Stock 3.31/3.27?

Since you're not the original owner (I assum) you haev no idea what could've been done to the car tune wise or gear wise.