...aftermarket cat back exhaust? i have been told that backpressure on an engine robs power causing the engine to work harder to push out exhuast gases. so if this is true then why did my truck get worse mpg when i installed a free flowing aftermarket cat back exhaust system? it was a generic flowmaster muffler called mellotone that is single inlet with dual outlet ran out the back of the truck. Before the install with stock exhaust i got 15+ mpg, with the cat back exhaust i now get 13 and my driving habits have not changed i have had a watchful eye beieng kept on me by my wife to tell me this. also my truck is a 99 f150 4x4 with a 5.4 triton V8 with a K and N air filter which was installed months before the exhaust.