Venom my dd situation is about 27mi each way on the highway, I have the Toytec ultimate 3" package and 285/70/17 BFG KM2, as well as a large safari rack with Hi-Lift mounted to it.
That being said, according to onboard computer, I am getting 16.9 mpg. In actual miles, I guess I drive about 375mi per tank - 20gl, per week, so that calculation gives me just over 18mpg. Probably somewhere in the middle is the happy medium.
I am driving to Colorado this week and will be taking more accurate measurements and will include somewhere in my write-ups during my holiday 4 wheeling extravaganza. Good luck with the tire purchase!