Quick question!

42441 is 2.25 inches but not the right design for the stock catback. For the stocker, you want part # 42443. 2.5 are a little bigger than the stock 2.25, so if you choose that route, the muffler shop doing the install will have to make it fit somehow...IMO its not worth anything hp wise to go larger with the stock pipes
 
GregGTStang said:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=7908544818&category=33636
Is the part #42441 for a Mustang GT? I also need to know if the 2.5" would be okay on my stock piping?

nah 2.25 is stock piping, the american thunder after-cat (Catback) system is done in mandrel bent (no crimps in the pipes) 2.5s.

you COULD get the 2.5s to fit, but it would be messy, and may require more work and time. If you're keeping the stock pipes, use the stock pipe size.