Go to autozone. There should only be two different sizes on the shelf. Get the bigger of the two. You'll also have to buy a coupler kit which connects 3" down to 2". You'll only need the 2" piece which is the only reason to get it. Go to the next isle over (exhaust isle) and get a 2"-2 1/4" exhaust adaptor. That's all you'll need. The rest I think you should be able to figure out. Good Luck!