So, if I put on a tall pizza cutter with a long haul highway tread I shouldn't see much of a mileage hit.
I expect a heavy duty tire with high psi and load, and low speed rating, to have high rolling resistance and high fuel consumption. That's why I thought the LT would be better, not worse! The commercial tires are made to haul a lot more weight than you need... that's the issue. Maybe that is something you can find data on... ?
Contrary to popular belief, larger tires with the same construction and compounds tend to have lower, not higher Crr. Larger diameter, greater width, lower profile are good for Crr. Hysteresis is the problem; how much internal friction is generated as the casing distorts and springs back. Speed ratings are temperature limited so that can give you an indication.