I wanted to post an email I get from a tech at Goodyears a couple of years ago as food for thought. I always heard that there was no standard rating for the ply rating of tires. His email helped confirm that.

Per your email question let me first address Ply Ratings and ATV
tires. There is no formal testing guidelines establishing
requirements for ATV Ply Ratings. Note that some manufactures
market their ATV tires with Ply Ratings but often times this
has no correlation with the number of plies in the tire. One
guideline is to determine the star rating 1 Star (4psi), 2 Star
(5psi) and 3 Star (7psi) for a tire and Goodyear Dunlop have
equated these different ratings to 2 Ply (1 Star), 4 Ply (2 Star)
and 6 Ply (7psi).

Therefore to answer your question we market both Mud Runner and
Rawhide ATT as 3 Star (7psi) or 6 Ply rated. This is not
stamped on the sidewall of the tire.
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