If it has any effect on your MPG it will be very miniscule. Depending if you hit the gas harder or take off a little harder to hear the exhaust then the mpg could go down. If you don't, then it'll stay the same or maybe just slightly improve (Barely noticable) but it could up the mpg.
1999 Chevrolet Camaro SS (PC'd)
1994 Chevrolet Beretta Z26 Q4 (W41 M90'd)
2004 Chevrolet Silverado 2500 (4WD)
My wife's cars:
1971 Dodge Charger SE (stock and original 383 auto)
2004 Subaru Impreza WRX (kid-hauler)