Max wheel and tire combo.

Ok, that makes more sense. A 275 is tight even if the backspace is perfect, on some cars some quarter lip modification might be needed depending on your set up. With BBP axles the math answer is usually about 5.25" of backspace. But, since you're running 255's already with 8" wide rims and a 5.5" backspace we can probably get closer than that. How much clearance do you have to the quarter lips and inner tub now?