without knowing what the weight each tire is going to have to carry, and what weight they are rated to carry, there is no way to know what pressure they need.
Ideally, weigh the vehicle, add the capacities of all tires, divide the total weight by the total capacity to get the percentage of capacity you will be supporting. Then multiply the maximum pressure (in this case 80 psi) by the percentage.
And yup, pump 'em up to that in the morning while they are still cold and don't change the pressure when they heat up in service, as that temperature rise is already built in to the pressure they stamped on the tire sidewall.
If you pump 'em up to 80, and no doubt have less than the full capacity on the rig, you might gain a little bit of fuel mileage, but it will be at the expense of a harder ride and unless on radial tires, the center of the tread will wear out prematurely.
