Tires: Standard Load vs. 6 Ply, passenger car tires, synthetic fibers

Question
Why would a standard load passenger tire and a light truck 6 ply, both 265/75R16, have different weight capacities?

Answer
Kim,

Yes,  Passenger car tires are sometimes used in lighter duty truck service and they have different load carrying characteristics compared to tire dedicated to that type of service - LT metric tires.

For example:  A P245/75R16 SL has a load carrying capacity of 2271 # at 35 psi - but only if the service is a passenger car.  If the service is a light truck, a van or a trailer, the load carrying capacity is reduced by 10% - to 2065 #, still at 35 psi.

An LT245/75R16 Load Range C (formerly known as 6 ply rating)has a load carrying capacity of 2205 # at 50 psi.  However, at 35 psi this tire has a load carrying capacity of 1700 #.

BTW, the reason "Load Range" is used nowadays is to prevent confusion about the actual number of plies in a tire.  " 6 ply" - meaning 6 ply rating - seems to imply that the tire ought to have 6 actual plies.  When tires were made from cotton, this might have been true, but modern synthetic fibers - polyester, rayon, steel, etc. - all have much higher strength, so less of it can used with equivalent strength.