What is the largest tires I can fit on my 03 FX4 rims? I have 31's on it rignt now, how big can I go?

 

The largest tire you can go with is a 315/70-17 (35x12.50). However, they will be extremely stuffed and will rub the frame slightly at full steering lock. Personally, I would stay with a 285/70-17 (33x11.50), 305/65-17 (33x12.50) or a 295/70-17 (34x12.00) unless you plan on lifting the truck.