It's not just the diameter of the pipe but the bends also, as well as the type of muffler. I use 2.5 with Walker Turbo mufflers, tail pipes comes out in front of the rear tires (none over axle). This is my van set up with a 70' 340. Note: Being a van, the collector is 2.5". I welded flange to the pipe, then attached to collector, by bolts. I noticed a huge difference between 2" and 2.5".