Tires: sidewall plys, passenger car tires, heavy duty truck tires

Question
what is the minimum ply of a passanger tire sidewall

Answer
James,

You may not believe this, but the answer is none!

Plies are an aid to minimizing the cost of manufacture of a tire.  The casing could be made out of thicker rubber and be just as strong.

The number of plies is almost immaterial to the tire because you can always use a stronger cord.  For example, heavy duty truck tires that use over 120 psi have only one ply.  It's made of steel, but it is only one layer.

In passenger car tires, it is common to use a single ply of polyester cord for small sizes. Larger tire sizes tend to use 2 plies, so there's obviously a transitional area.  But in theory, even the larger tire sizes could be made of a single layer, but a larger cord.