Hi guys. I'm a total newbie, both to diesels, and to this forum. I also wonder what's up with my truck. I have a 2003 F250 6.0 crew cab 4X4, automatic tranny, 56,000 miles with 285/75-16's. 4" aftermarket exhaust. Stock Tune, and a tonneau cover. I keep hearing I should be getting 20 MPG. I've...