I have seen alot of conflicting information on here about exhaust diameter so I went out and measured myself. All of these measurements were done with a digital caliper. These measurements were done on an '03 X and may differ from yours. This may be old news, but I just wanted to clear up the confusion.

The after-cat piping is 1.75" and becomes 2.25" at the "Y" all the way to the muffler inlet. The muffler outlet is 2.15" and the piping read anywhere from 2.12"-2.15" all the way to the tip. The tip is 2.5".

I don't have Photoshop at work, so I did a quick MS Paint diagram for those wanting to see.