Without adding spacers in the rear, I have done 235/80R17s. Leaves about .5" between the tires. I don't like to get much tighter than that. Chance of heat buildup and blowouts if tires are touching. But if you want to run a 1.5" or 2" spacer between your duals, you could easily do 285/70R17 with some cranking, trimming, etc on the front.