Ok what is the max PSI for 315/75/15D tires? everything I can find on the interent says 65psi but the sidewall says 50psi. So I have been running 50psi but if I can run 65psi pulling the trailer I will.
The rubber manufactures tire book from many years back allowed +10% for smooth interstate runs. That would be 55# - about the same as the tolerances on gauges.
The correct pressure can be found with chalk - make a stripe across the tread in a large parking lot and roll forward until part of it wears off. If it is the center drop the pressure a couple #, if the edges add 5# and do it again. When it wears off evenly you are within a couple of # of the correct pressure, if it is more than 10% less than the sidwall get the minimum pressure from the tire manufacture. Tires rated D and heavier are subject to 'zipper' failure, that is a very smooth break at the sidewall and tread joint about a foot long. A sudden let down at 80 in the hammer lane is not real entertainment. It also comes from overloading with one dual low on air.
One thing is always acceptable is the pressure on the sidewall.
so far all that i have had fail were under inflated, ran hot, and sidewall failed. Too much PSI can cause the same problem but is less apt to happen than under inflated / over loaded. As John said stick to the PSI / Load chart from the manufacturer and if you are going to error do it on the high side. Keep in mind wheels have a max PSI also. I run a 235/85 16 tire that calls for 110 psi at max load. That is 10 psi more than the wheels are rated for. A wheel failure wouldn't be pretty either. Keep in mind all inflation pressures are at a specific air temp and would have to be adjusted up or down based on true tire temp.
Do NOT adjust tire pressure based on increased pressure from temperature caused from running.
If attention to detail is your thing there is ambient temperature correction IIRC about 1 # for 10 F, so checking the tires at 80 F before a storm blows through and drops the temp to 0 F would change the tire pressure by -8 #.
The loss of 1 to 3 # pressure a month is normal so about 90% of the tires on the road are underinflated and almost all will have been run underinflated before the tread is worn out.
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