It isn't weight... it would be easy if it was, we'd just need to put them on a scale! My tires weigh 75 lbs/ea. The stock BFGs with street tread weighed less than half that, but MPGs were worse at low speed. It's hysteresis when the tire flexes; how much energy gets turned into heat.
Unfortunately no one seems to test AT/MT truck tires for MPG so there is little incentive for manufacturers to care... and it's impossible to tell. Generally you would expect a tradeoff with weight (for the same size) since that indicates more/thicker material, and durability since that indicates a lack of suppleness... but design and rubber compounds can make a huge difference.
One thing we can look at to get an indication is speed rating for tires that have the same weight rating, since that is primarily a temperature test. Higher temperature (lower speed rating) means more heat generation from hysteresis.
I haven't managed to destroy mine yet, but I do have some pretty deep sidewall cuts. And my spare is an ST Maxx...