I have a slusho automatic; stone stock getting 24-25 mpg on the highway with stop and go.
run 42 psi in your tires, easy starts and coast to stops, make sure car is tuned up and keep it waxed... waxed has proven 1-2 mpg difference.
Speed is the killer. Keep it at 65 on the highway.
Usually once a tank you crack that biznatch WOT to ensure she's cleaned out.
If you're stick, you should be able to squeeze 27-30 out of your car, as others have bragged on here. My best fillup was 25.00 mpg even; with 70 percent highway, 30 percent city.
I could really care less if some tart in a minivan wants to blow past me at 80mph or the rest of the big square boxes out there on the road. I believe the Mustang has sole that the rest of the tupperware appliances out there fail to deliver, hence the reason the faster they get home the better.
I enjoy the drive! After all, that's why I bought a GT.