Took my truck on the first trip after putting in the 6.2L and worst case I got 17 mpg going over the mountains agains a strong (20-30 mph) headwind. After that I was getting 18.5. No heavy loads on it or trailer this time out but still much better than the old 350 ever did. I'd probably been lucky to get over 10-11 mpg fighting that headwind over the mountains with the gasser. Sounds like the motor wants to come apart over 50 mph but mainly because I have no carpet or floormat in my truck right now, just a bare steel floor. I'm also missing a spring off of one exhaust manifold to pipe flange that I need to put on tomorrow that will probably help too.