^ Figure your axle weights and then use a load/pressure chart for your size of tire.
You can look up your exact truck, but I know your truck has a curb weight around 6000lbs and it's about 3500lbs FA, 2500lbs RA giver take.
So being conservative (lower pressure = better traction and reduced high speed handling, I call it "winter air" and the opposite for "summer air")
Say truck is 4000lbs FA = 40psi min pressure (2130 lb tire capacity at 40psi)
3000lbs RA = Probably around 30psi min (35psi = 1930lbs capacity)
Add trailer, say you add 1000lbs to the rear axle.
RA = 5000lbs = 50psi for 2535 lbs capacity per rear tire.
This is not needed to be calculated to the lb or exact 1 psi, but rather a starting point.
If it feels a little squishy, air up a little from the min.
If it's riding like a lumber wagon, air down some.
Way too little and you're killing mileage and your tires, way too much and you're not getting as good of traction and beating up your steering and suspension.
Case in point.
I have a 3/4 ton diesel. Previous owner had relatively new 37x12.50s on it. He had both front and rear at 65psi. Doesn't sound too bad, right?
Wrong. Tires had low miles, rode like they were made of wood and had worn down the center of the tread about 3/32 more than the outside edges which had 0/32 total wear. And it handled worse because the contact patch was slow, tires didn't soak up bumps and they tracked bad in the wheel ruts.
I am running about 45 psi front and 30psi rear, empty. Add 1500-2000lb tongue weight, I run 45psi front and 40-45 psi rear. WITH almost a ton on the hitch. Truck drives much better.
