First what type of gas mileage increase should a flow master cat back give me? It has an 80 Series muffler in the kit? Second what performance gain on a lesser note.. Note my cutout was always open and I am selling it and it leaked like a siv around the cutout.

 

You probably won't see a difference in gas milage.

You however will see a decrease in performance. Somewhere probably around 10-20 rwhp by going from a cutout to a Flowmaster muffler.

 

No it is an entire cat back.... The 80 series is just the muffler it comes with.