I would like to keep my stock wheels but I am thinking about doing a 4 inch lift on my 350. I have the factory aluminum wheels. What is the biggest tire size I can safely run on my stock wheels?

 

The only dimension that is affected by wheel width is tire section width. The height aspect ratio (255/60 vs 255/85 for example) does not come into play. Manufacturers set tire width specifications. 255 is about as wide as you can go without going beyond spec. Folks I have come across have gone as wide as 315, I have run 285 myself with no ill effects, in fact I still have two 285/75 18 LT on the back waiting to wear them out.