86 C-10 max backspacing for a 6" static drop
 

We lowered my neighbors truck in the front tonight another 3 inches with springs. He already had 3 inch drop spindles.

Here is the question......what is the max size tire and rim size for a 20" rim including backspacing? He currently had a 15" x 10" rim (4" back spacing) and a 275 x 60 x 15 tire. The top of the fender rests on the top of the tire.

He plans on buying a set of 20's.

Thanks