My Avons are 12.5/27.0-15, and the tread width is actually a little more than 13.5" wide. They are on 15" BRM style wheels, by Vintage Wheels, iirc. The wheels are actually 13" wide, lip to lip (I measured them).
BTW, I also have a set of 17" BRM wheels, same size, that have Michelin Pilot...