So which is better overall. the 3.55 or the 3.92 Axle?

 

I got the Extreme. It sounds great. Probably a little louder than the 40. It hasn't been on for long enough for me to get gas mileage info. Iwas getting 10mpg around townwith my FM 50 SUV.Right now, I'm hovering around 12mpg. I've gone through 1/4 tank. The difference between the FM40 andExtreme is that the FM is a 2 chamber muffler and the Extreme is a 1 chamber. The Extremeis also cheaper. Its similarto Flowmaster's 10 Series race muffler, except you can get itdual out (FM 10 only single out).