I have had my oil changed at the fast lube and they filled my tires to the maximum psi on the tire sidewall rather than what was called for on the door placard.
I have found the best way to set tire pressures on oversized tires is to place a chalk mark across the tread section, and then drive for a bit on a hard surface road. If the chalk mark is worn off evenly, your pressure is about where it needs to be. If it wears off more in the center of the tire, the pressure is too high. If it wears off more on the outside edges of the tire, then they are under inflated.
When I ran 37 - 12.50's on my Ram 2500 Cummins, 50 was good in the front and about 38-40 in the rear. If I hauled a trailer or load, I increased the rears to 50.