Tires: tires, inflation pressure, wheel position

Question
who to determine what ply a tire is,thanks for your time,

Answer
Thomas,

I am not sure what you are asking.

If you are asking who determines how many plies of material are actually put into a given tire, that would be the tire manufacturer.  It is determined by internal design standards each tire manufacturer has based on the tire size and the maximum usage inflation pressure for each tire.

But if you are asking who determines what PLY RATING is applied to a vehicle, it is the vehicle manufacturer.  They determine the tire size and ply rating (also called Load Range) by the load being carried in each wheel position plus some additional factors unique to each vehicle manufacturer.  They then consult reference books published by tire standardizing organizations which list the tire size and the relationship between load carrying capacity and inflation pressure.  The tire manufacturers use this same book to determine the construction of the tires.