I didn't notice decreased gas mileage...you probably just get on the throttle more now. Could be the fact that you need to give it more gas to stay at a constant speed because the RPMs drop faster.
__________________
2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
Team B.O.B.® - Ballaz on a Budget