I read once on a 4x4 forum where guys are running oversized tires to get some chalk and a level, straight aspalt road. Set your air pressure to what ever you want on all four tires (32, 38, 40, whatever...). Using the chalk, draw a thick perpendicular line with across your tires. Then drive your rig as straight as possible as far as possible.
You'll see where the chalk runs off first, and you want to see an even wear pattern across the entire tire. Adjust pressure as necessary. If it wears out first in the center of the tire you're air pressure is too high. If it wears on the outside first then the air pressure is too low.
Try it maybe you'll find the optimum tire pressure based on your individual vehicle needs and maximize the lifespan of your tires.