I'm sure 80 pounds would be way too much for an unloaded truck. Since the tires are slighly taller than stock put 5 more pounds than stock. Draw a line across the tread with chalk and let the truck roll a few feet. Check the chalk line. Add or let out air as needed.
The chalk test is helpful. Just to clarify for folks that aren't familiar with this, what exactly are you looking for? I believe you want uniform smudging of the line. Uneven smudging would be more at the edges than the middle of the tread or vice versa. Can you remind me when you air up or down? I think more air if you have smudging at the edges and not the middle and less air if you're smudging in the middle but not the edges?
FYI, you can also just call the manufacturer. Oftentimes they will give you a recommendation for your specific vehicle and tire model. The sidewall pressure is max pressure. Unless you like the feel of driving on bricks you definitely don't want to run max pressure.
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