Do not run LT tires at a lower pressure than P rated tires. LT tires must be run at a HIGHER pressure to achieve the same load rating as an equivalent size P tire. Obviously, max load rating is achieved at max pressure (typically 80psi for load range E). You don't need max load rating, but you should equal or exceed the load rating of the factory tires. The following link tells you how to pick minimum inflation pressure for your P to LT swap. You can go higher but not lower. Also, be sure your valve stems are rated for the pressure.
P to LT Tire Info I ran LT tires on a 1/2 ton when I did a lot of dirt road work. They are heavier, give lower MPG, and may last longer due to the thicker tread. They are also more expensive and not generally needed. I always felt better with them but they will bit increase the load carrying ability of your truck.
