That is interesting, I wonder if the pipe from the CAT to the Y is 2.5"? I went to the Gibson Website, and they have very little information on it either. Their notes for the 05/06 system say 3" tailpipe, 3.5" Tip! Nothing about the size of the pipe from the CAT to the Y! So, maybe it is 2.5" to the Y then 3" from there back!