There has been lots of discussion on exhaust sizes and how much each extra inch in diameter helps at various horsepower levels. Many people just keep their 3" exhaust and strait pipe it. Some go to 4" flow-through mufflers. Some go to 5" flow-throughs, and a few even go to 6" and 7" setups. IMO, a 4" flow-through muffler or strait pipe system is plenty of exhaust for most people on here. Some need to go to 5", but I believe that anything over 5" is just more expensive. Going from stock exhaust to a 4" Flow-Pro, I dropped about 300* EGT and gained about 2 MPG. I'm not sure on the horsepower gain since I haven't been on a dyno, but I think it's one of those things that helps more if you have more horses. Feel free to comment and/or add your own experiences.