RE: 'Ply Ratings'
As far as I've been able to determine the "Ply Rating" system is no longer
in use and hasn't been for some years now.  It was a throwback to the old
bias ply tires but was not a good or accurate way to describe radial tires
that can be constructed a number of ways. Here is a page with a table that
shows corresponding Ply vs the more current "Load Range" info:
<http://flashoffroad.com/features/Tires/loadrating.html>
and below that the more precise "Load Index" ratings.

I haven't bothered to look but is anyone aware of any modern (current
production) tires that even have a "Ply" number on the side?