I went to chevy.oldcarmaunualproject.com and looked up the inflation charts and weight charts. The 3600 tuck had 15-6 and 15-8 tires listed. The 3100's used the 6.70-15 though. I'm guessing that the 7.00-15 were installed out of convenience more than anything else. I'm in the same boat with my 54 3600. It came with 7.00-15's also. I've upgraded to modern 16" rims on mine though. I'm still debating on whether or not to go with Bias or Radial tires.
