Originally Posted by dyn085
This is a misconception about exhaust systems. A backpressure increase will not help an engine get better fuel mileage due to the fact that the engine now has to work harder to remove the spent gases. Creating as close to a near-zero backpressure system is the way to go in order to minimize pumping losses and maximize the scavenging effect.
Obviously, this is the most retardedly short version due to the fact that there are multiple variables in creating the perfect exhaust. Not coincidentally, the perfect exhaust for fuel mileage is not going to be the same as the perfect exhaust for power or even for sound.
It's interesting that you say you lost mpg's by replacing the muffler, as my dyno diagnostic work has shown that power is not affected by it's removal. Judging by the timeframe of your post, I would be more willing to bet that your replacement occurred around the time that fuel stations went to a winter blend.
His mileage more than likely dropped because he keeps romping on it in the belief everybody enjoys the the sound of his fart can muffler.