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It might be time for a lesson in the difference between P metric and LT metric tires.
Small vehicles use P metric tires (or their European or Japanese equivalents). These tires are low pressure tires and this results in a need to make the rubber flexible enough to deal with the deflection required by the load / inflation curve. Low pressures also result in good ride qualities.
Larger vehicles (meaning 3/4 and 1 ton pickup trucks, vans, and in particular the Grumman trucks discussed above) use LT metric tires. These are high pressure tires ("high pressure" being a relative term). These tires tradeoff good ride qualities for increased load carrying capacity for the amount of space the tire occupies. This also makes the tire more efficient from the material / load capacity point of view. An example of this sort of tradeoff is aircraft tires where space and weight are at a premium. Some aircraft tires use over 300 psi inflation pressure.
This also results in tires that are more potentially retreadable, where P metric tires are less so because of the amount of flexing that takes place.
Which brings me to what is printed on the sidewall of tires:
There are government regulations that specify what is to be imprinted on the sidewall of tires. However, the US regulation is a little unclear. While there is agreement within the tire industry as to what the regulations say about maximum load, there is some disagreement in interpreting what pressure is supposed to be imprinted on the sidewall. Notably, the Michelin group (Michelin, Goodrich, Uniroyal) seem to think that S and T rated Standard Load passenger car tires should have 35 psi imprinted, while the rest of the industry thinks the regulations say that higher pressures should be imprinted (since it is permissible to use them.)
Hence, you will find tires that are fundamentally the same (same size / speed rating) with different max pressures listed on the sidewall.
This problem doesn’t exist for LT metric tires, so there is complete agreement within the tire industry as to what gets imprinted – both for load and pressure (if you take into account the difference in tire standards).
But there a small amount of overlap between passenger car tires and LT tires. You will find tires with the same "size" (meaning physical dimensions) of both types. The problem here is that the load carrying capacity of LT metric tires is lower than the "same size" P metric tire - due to the difference in the allowable deflection in calculating the loads. My take on this is that the tires are not interchangeable.
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