I'm running 285/75/17, basically 34x11.50, Toyo MT's on my 4runner with stock 4.30 gears. These are heave at about 70lbs each. I get 14-16 mixed and can get 18 on highway alone. I think you're geared too deep for your tires. I switched from 265/70 to 265/75 and didn't notice any difference...