My G isn't very sensitive to load, but is to low speed. If I'm in high range doing 15-75mph I'm seeing 10-12mpg, but if I'm on difficult trail in low range doing 3-6mph, I'm averaging about 3-6mpg.
Mine should be less efficient than most though, it's on portals, and I run 38" tires aired down quite low when on the trail, 5-10psi low, plus it's an AMG so I have the supercharger making it use more gas as well.