what is the biggest size tire that i can put on my 2003 ram 2500 with stock 17x7.5 rims?

 

285 is the widest width that would fit on that wheel. The 285/65 would match the diameter of the factory optional 265/70, but that's a very uncommon size. 285/70 fits on the wheel, but might rub unless you're lifted, because it's wider and taller than stock.