What should the air pressure be on 31x10.50R15 tires?

Air pressure is dependant on the vehicle NOT the tires. If you had a 1970's Jeep CJ 4 cylinder, it might only need 24-25 PSI. But if you had 1 1980's Blazer full size, it might need 35-41 PSI. And it could be that you try to put it on an Excursion (not actually possible, but for discussion sake only) and you couldn't safely put ENOUGH air in it!