what is the biggest tire i can go with to clear 20" rims with agressive all terrain tires on a 2001 supercrew 4x4?

 

I'd suggest a 305/55-20 (33x12.50) tire on a 20x10 inch wheel with 4.5 inches of backspacing. You will have to trim a little plastic off the front bumper's lower valance to clear this combo. I have some pics of this trimming in my gallary with 33's on 16x10's.