Originally Posted by piranha971
Here is what NOT to do if you are looking to improve fuel economy: get rid of the rear muffler.
Leave it alone. Its there to create back pressure and improve fuel economy and doesn't do much muffling. I would say replacing just the rear muffler with a straight through Borla cost me 2mpg with no noticeable difference in performance. The car sounds right, though. I just could not stand the total silence of the stock exhaust..... And that ugly pipe in the back.
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.