Andy,
6.00-16 (4.5" rim section), 6 ply rating tires were standard (16" rim)
6.50-16 (5" rim section), 6 ply rating was a common factory option (16" rim)
15" (5.5" rim section), 6ply rating was also an option in 1954 (15" rim)
The rim section, I think, is the width of the rim at the lower inside edges.
You can put a larger outer diameter tire on a rim (within reason; maybe up to a 33" loaded outer diamter) but the inner diameter of the tire must match the rim diameter. If I am wrong, we can be sure that someone will post a correction.
Tim
