I think changing the rear gears and, if needed, lightly modifying the engine to lower the torque curve will make very significant gains.
I remember the '93 3500 we had, i actually drove it in high school. It didnt matter much how you drove it, it never got more that 13 to 15 mpg. 2bbl tbi, no tach so who knows what the engine speed was. Had absolutely no trouble running any speed you wanted no matter what you had it loaded with, massive amounts of torque.
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