The stock size is 2.25 inch. The best route I feel would be go get a new 2.5" cat-back section made if you buy that muffler. I recently had a Magnaflow 12148 installed which is basically the same muffler with the stock 2.25" in and dual 2" out and had a new cat-back made so that I could keep my stock system intact for later if I didn't like it. I'm kinda glad I did because the local shop that did it made a lot of crazy bends and made a restrictive system, but I plan on buying the Magnaflow cat-back sometime when I get some money. The sound is amazing though.