On the 55 1st series 3/4 ton the standard equipment was 15inch 6ply rated at 1500 pounds each capacity. Optional was a 7.50-17inch 2100 pound rated for single rear axle. 6.50-16inch was listed as an option for front and duel rear 3603 only. I don't know if that was the same for the earlier, but I have always seen the 15inch listed as the standard sized wheel on Advance design 3600s.