The reason larger tires generally result in lower MPG is due to people usually choosing WIDER instead of TALLER tires. To increase the width, you also increase the contact patch, which increases the road friction... Which increases the demand for power to counteract the friction. You can counteract the increased weight of the larger tires usually by just cleaning out the dead bodies stacked in the cargo area. Most of the cars I've seen around here (grad school) are rolling landfills. Mine too, sometimes.
That's why I went with the 215/85 size - Narrower contact patch, and I can lengthen or reduce that patch by changing the air loading in the tire. The tires are also TALLER than the stock, so the wheel turns slower at the same road speed - less revolutions = less fuel spent for the same distance. The 215/85 fit just fine without a lift kit, the next set I get will be 235/85, which would not fit without at least a 1" lift, probably a 2" would be best. The 215/85 are just over 1/2" taller than stock, the 235/85 are 1.5" over stock, and reduce the rpm by a BIG amount.
Since I do mostly highway or on-road, the reduced width doesn't bother me, and the KJ still has the ability to pull stuff out of the mud if needed, just by airing down a LOT to soften up the tires. Since that isn't my primary duty... I'm OK with having to change the air once in a while.
YMMV of course.