[Linus Gump]

Could it be that with the H3 only having a 3.5L engine and trying to move 4700 lbs of vehicle with taller tires is more taxing on the engine than a 6.0L (or whatever is in the H2) trying to move around a slightly heavier vehicle with taller tires, but not as taxing to the engine?

Seriously though, the little H3 is overworked to begin with, and the addition of taller tires makes it have to work just that much harder. That extra work comes at the cost of the fuel economy, whereas the H2 has power to spare and can handle the taller tires easier.