If on a 1/2 ton truck around 35 PSI is a good place to start. With some experimenting you may need to tweak that up or down. When running factory P series tires on a 1/2 ton 35 PSI will give the best ride, but the sidewalls suggest 44 PSI for heavy loads. I always ran closer to 40 PSI with P series tires. The ride was a little stiffer, but I got better fuel mileage, didn't have to air up when I hauled a load, and still got 60K out of a set of tires. LT tires are stiffer and actually need LESS air to haul the same weight. The tire store put 50 PSI in mine when they mounted them. About right for an unloaded 3/4 ton. But I found the best ride at about 30 PSI. With the LT"s at 35 PSI they ride about the same as P's at 40 PSI and will carry more weight than a 1/2 ton is rated for at 35 PSI.
