[BroncoAZ]

Your stock wheels will work for any of the tire sizes listed. I ran my Bronco for a year with 32x11.5 on 8" wide wheels (4" backspace) when it was stock. The size worked well. Next I went to the 33x12.5 on the same 8" wide wheels. They worked well and fit fine stock. 10" wide wheels cause tires to rub. Each time I went bigger the fuel economy went lower. I think the 33x10.5 might be an ideal size. The extra width of the 12.5 definately doesn't help the mileage, and the 10.5 would clear the radius arms for tighter steering. BFG's always worked well for me in everything except deep mud.

I'm shopping for tires for my diesel right now. The stock tires are a 265/70/17 (31.4x10.5). I'm looking for something in a 33x10.5R17 with an E load rating. The closest things I'm finding are a 285/70/17 (32.8x11.5) or a 275/70/18 (33.1x11). Taller tires will give it better gearing for going 80 mph on the freeway, but I don't want to go wider because it will mean more rolling resistance, or lower fuel economy.

 

[BroncoAZ]

Your factory rims are 4" backspacing, so they are fine. Some tire shops have an issue mounting a 12.5" wide tire on the 10 hole aluminum wheels because they are 7.5" wide.

I couldn't quantify the exact amount of gas mileage lost. The best my bronco ever did stock with 32" tires was 16 mpg. After going to 33" tires and later a 3" lift it dropped to 14 mpg or so. With 35" tires, 4.10 gears, and a 4" lift it was getting 13 mpg. Currently with a modified suspension, 35" aggressive tires, and 4.10 gears it is getting about 11.5 mpg on the highway. Since I seldomly drive it, I really doesn't matter what kind of mileage it gets.

If the tires are too tall the truck will fall out of it's power band on the highway, making the engine work harder. The wider the tire the more rolling resistance makes them harder to turn. Both of these factors make it use more fuel.