I agree with Jim Sears. Tire pressure is a function of load. In sports car circles they say that tires should gain about 3 psi after running a few miles.
My rig carries 6,000 lbs on the rear and 3500 lbs. on the front. If I inflate the E-rated front tires to 80psi listed on the sidewall, it rides rough and handles miserably. I run 36 ft. and 70 rr. when loaded.
Last weekend I drove it pulling a utility trailer 250 miles, left the trailer and came back empty. I was too lazy to drop the pressure on the rears and suffered from poor lateral stability. The saving grace was the trailer acting as "drouge" and keeping things going straight. Coming back empty it was a lot more work to drive. I almost stopped to drop the tire pressure to about 36 on the rear but then the road improved and I just suffered with it till I got home.
