I'm just adding my 2 cents in, I have the 4.10s, not the 3.73s and the mpgs I lost was little, and mostly noticable from HWY driving. The 3.73s are probably a little better, maybe not noticable. Also, I have a Automatic, so that too may make a difference.
07 GT. PREMIUM SCT: CUSTOM TUNE by "MUSTANG SPECIALTIES". FULL MAGNAFLOW COMPITION EXHAUST, LT. HEADERS AND X PIPE, LMR: GEN2 2010 STYLE HEADLIGHTS, RAXIOM: 2010 SMOKED COYOTE TAIL LIGHTS, WHEELS: STAGGERED WHL 22X8.5F-22X10R MORE LISTED INSIDE.