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 SCT: CUSTOM TUNE by "MUSTANG SPECIALTIES". STEEDA: CARBON FIBER HIGHFLOW CAI. STEEDA: 1 PIECE ALUMINUM DRIVESHAFT. FRPP: 4.10 GEARS. MAGNAFLOW: AGGRESSIVE 3" 409 STAINLESS STEEL DUAL EXHAUST. BBK: LT 304 STAINLESS STEEL HEADERS, BBK: o/r X-PIPE. MSD: BLASTER PERFORMANCE COIL PACKS. POWERSTOP: CROSS DRILLED SLOTTED ROTORS. WHEELS: STAGGERED WHL 22X8.5F-22X10R.