This weekend I chopped off the muffler on my 1999 1500 5.3 Silverado and I drove a couple hours back home and gassed up. I was deplored when I figured out that I did 16MPG (Canadian). It usually does 19-22 MPG on the highway at a steady 120Km/h. I did place a piece of exhaust pipe to replace the muffler so It is exhausting at the tailpipe. Wasn't driving in abnormal conditions or anything. Its never done that bad before this. Is this that back pressure problem?