Make sure they're aired up properly, and no, that doesn't mean the psi marked on the sidewall of the tire. Also make sure that no brakes are dragging. All in all, you shouldn't have too much problem with 31x10.50r15s. They were a factory size on some.
I would suspect probably less. What's the weight rating of the tire? It'll be printed on the sidewall of the tire as x lbs at x psi. So, given 4 tires at x weight rating at x psi and your vehicle weighs x lbs (you can find that on the drivers door jamb sticker), you should be able to find the psi at which the tire will properly support your vehicle. This will give you a solid footprint without being under inflated (which will reduce gas mileage and cause undue wear) or over inflated (which will reduce your overall contact area with the ground and not give you as good as traction as you should have).
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