Jon, I'd have to say the size that Coker quoted you is incorrect if you are interested in "original" size. The size they quoted you is a metric radial which as we all know hadn't even been developed in 1957. According to my 1957 Dealer Data Book
the following 15 inch size was available for all 3100 series trucks: 6.70x15 in either 4 or 6 ply rating. A 6.50x16 and a 7.00x17.5 were also available. The 6.70x15 was indeed offered in a whitewall. The only model in the book displaying the whitewalls is the Cameo and they appear to my best guess to be about 2 to 2 1/2 inches wide. Coker has these sizes I'm pretty sure so you may want to contact them again with this info.
