I've sure thought about it with a 95 F350 flatbed dumptruck I use. I get 5-6 mpg every day in that pig, and gutless as hell to boot. I don't think I could get 7mpg if I through that POS out of an airplane from 40,000 feet.
A Cummins would sure make a good truck out of it. Only reason I haven't done it is I have a pto for the dump body running off the Ford auto trans and I wouldn't want to keep the auto. Don't want to lose the pto as it cost a couple K when I had it installed.
