The manufacturer will have recommended PSI in their catalog. If the tires aren't made anymore, old catalogs are easy to find with a web search, though very old ones, 10+ years get harder to find. Or, find a tire similar and see what they recommend for that. For instance, look at the 12R20 XZL here: http://www.thebox.fr/Michelin/XFORCEMILITAR040416.pdf
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