I just got finished installing my new magnaflow high flow cat in place of my stock clogged cat. It is quite a bit louder than my old one (the exhaust ends out of the end of the cat; no mufflers). Anyways, there is a small but noticeable gain in performance. I dont know about the gas mileage yet. The installation of the new cat was extremely simple. Fit right in place, and took about 10 minutes to hang. Getting the old one off wasnt too hard either. If you ever have to take one off, dont waste your money at an exhaust shop. I just cut the cat off with a sawzall. Then used a dremel tool to cut the remaining larger piece of metal off of the exhaust end, by cutting a line in it to make it resemble a "c" shape. Once I cut through, the old piece came right off. It was simple, and I am happy with the results. -Harrison P.S. If you do this, I know its hard not to, but try not to drive around in your neighborhood with open exhaust manifolds as it seems to piss some people off. Plus, im sure its a noise ordanance violation or something like that.