Changing just the muffler should cause very little difference either horsepower or FE. I've done a lot exhaust changes on cars over the years and have tested these on chassis dynos. To get a muffler to make any discernible difference you to need to do more to the system in before the muffler. Also every car is different. I had a 2001 Mustang GT that a put a set of Flowmaster mufflers on. Nothing. Next I put on longtube headers, X pipe with no cats, and a new cat back exhaust system picked up 30 hp. Next added a CAI and a chip. Got another 14 hp. When I went on trips I would set the cruse at 80 mph and still knock down 27 mpg. Not for a car EPA rated 23 highway. The car was making 265 rwhp so that equates to about 312 hp at the crank. Added about 52 hp at the crank but increased FE......go figure.
P.S. Before anyone bashes me this was in my pre-eccomodder days
