Reviving this thread to see others' MPGs who have done tire and roof rack mods. With FT4x4, heavier and bigger 275/70/17 BFG E-rated tires on SR5 rims, and Baja rack, I am suffering far more than I had hoped. Also, I have noticed that the AC (and blower fan) consume nearly 1MPG. Getting 15-16 in town, and only 18 on HWY. Over 60 mph, it gets uglier. Also, wondering if the increase in tire size has caused my MPG readout to run lower than reality. Even when I was all stock before my mods, I still could never top 20mpg on an average basis, unless I was going around 60mph on flat roads only.
I did do a 15-mile jeep trail in steep mountain gorges recently all in 4LO and got 13MPGs somehow.