Those stock mufflers are heavy as heck. Not sure the weight of each, but I would guess 30 pounds each? That is 60 less pounds and can potentially knock off .1 of a quarter mile if ya' took your spare tire out too. I took off my stock cats and stock mufflers and average 23.4 mpg city with a 5 speed.
2009 Mustang GT Premium; FRRP 4.10 Gears, J&M Lower Control Arms, BBK Long Tube Headers, BBK Catted X Pipe, Pypes M80 Mid Mufflers, Pypes Pype Bomb Axle Back, JLT Series 3 Cold Air Intake, Bama 93 Octane Race Tune, Black Decklid Panel, Weathertech Mats, SVE Shifter and Shorty Antenna.