There are two distinct rules you can follow here.
Most - not all - ST/Trailer tire manufacturers will recommend using inflation pressures molded onto the tire’s sidewall. They are age-out tires and will most likely do so before the treads wear out.
Tire industry standards suggest - with strong wordings - that OE tires be inflated to the vehicle manufacturer recommendations - almost always what the side wall says.
Replacement tires must have a load capacity equal to or greater than the OE tires provided. Plus sized tires should have an inflation pressure set at a value that provides enough load capacity to equal or exceed what the OE tires provided. However, you have wasted your money on the upgrade if you do not take advantage of the increased load capacity the plus sized tires can provide with increased inflation pressures.
Don’t short change yourself with tires that are not suited for your rims. All steel cased 16” tires require 6.5” wide rims, not 6”, squeeze it, or 7”, bulge it. Some say load capacity is the correct factor. Go and look at some SAE specs, they all include Load ratings & PSI ratings. (SAE certifies all of our trailer tire rims). Even the tire manufacturers are serious about rim widths. I took this picture of a 16" Westlake tire at a RV show. Its load capacity is 4080 at 110 PSI.......
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