Yeah 80 MPH with 4.56 gears I think you did really good to avg 10 mpg.... FWIW I have never seen a 360 get any kind of mpg at all. Everyone I have had was a total gas hog... Can't say for certain but I would think the HEMI would probably get better than the 360 but for that kind of money you could probably swap in a Cummins 4Cyl or 6cyl and problem would be solved...