45 is the maximum, and that would be around the pressure you might have to go to to seat the beads when you install them. But the 28 psi is the reccomended pressure for the M38A1, based on the gross weight. As Grant has mentioned, it will have to do with temperatures, but is also based on the amount of sidewall deflection (which is related of course). It also lets you get the most mileage out of the tread, although in your case it will be more likely that you will exceed the expiry date of the rubber before you wear out the treads on your truck.
Note that 28 psi is the maximum, so would be the jeep with a full tank, crew and payload to max weight. You could go a little lower for better traction, but really, like most of us I'll guess you are just cruising maybe a couple hundred miles a year during the spring/summer.
If you were to drive them around with 45 psi, I think it will be like driving with Fred Flinstone wooden wheels. The pneumatic tires add some (if not most) of the smoothness to the Jeep's ride.
I just set my tire pressures yesterday, all to 28 psi. It allows me to engage and disengage the 4 wheel drive without driveline binding.
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