I'll bet the tire manufacturer's minimum recommended tire pressure is 34lbs.
2 extra pounds for each 2in increment.
18" 30lbs, 20" 32lbs, 22" 34lbs.
But maybe the amount of sidewall could change this rule of thumb.
retail tire companys recommend as a rule thumb it should be -10psi the max lpsi load/ Air in tire will expand and contract with tempeture, NITROGEN doesn't expand/contract nearly as bad as just air. I have 20" and run them at 40 psi with NITROGEN..