Hey guys, just thought I'd drop by here, I'm Schwaebz's friend (the owner of the 99 Mustang GT).
I've got a 1998 Ford Mustang V6, I've done a lot to it, although gas mileage is a secondary thing to me, I have greatly increased it with my current mods.
Originally my car was a 3.8L v6. I have since stroked the motor out to a 4.2, overbore of about .03 and honed it, that alone drastically increased gas mileage as well as my vehicle's power. Other mods include, a high flow cold air intake, free flowing exhaust (No crossover) just straight pipes back from the cats into mufflers. My car is also lowered so it gets a minor aerodynamic airflow advantage. Lastly I've got 3 custom tunes from Brenspeed. One for economy (for long trips) and two that are interchangeable for performance. I use the SCT X-Cal 2 to change between tunes, I normally run the economy tune unless I'm autocrossing in San Diego, with the economy tune I normally get around 28MPG in the city and about 32MPG on the highway. Who says domestic muscle cars can't get good gas mileage??
My plans are to get underdriver pullies, a higher flowing fuel pump to less restrict the fuel lines, aerodynamic front and rear splitters, and some other modifications.