Dan: Your commented a while back in another thread that a G614 tire would be ruined with prolonged use with less than 110 psi. Is this idea with lowering tire pressure for load something new?
The prior comment about running less than 110 psi was based on a specific low inflation situation.
Goodyear says if you run any length of time at 20% underinflated the tire should be considered to have been "run flat" causing damage. The main variable underneath that is how heavily loaded you are. If you're close to the max weight on the tire and instead of 110 psi you're at 85 psi, you're more than 20% low.
But Goodyear also shows inflation tables for specific loads less than maximum. So if the trailer was empty, the tires aren't fully loaded. In that case, tires inflated to a lower value than the max cold pressure on the sidewall may be ok - per the inflation tables.
My understanding is that you're trying to avoid excessive flexing of the tire as you go down the road. The part of the tire in contact with the road is flattened momentarily, causing flex. If you're overloaded or underinflated, the tire flexes more. Flex causes heat. Excessive flex causes excessive heat.
High speed also causes additional heat from more rapid flexing. Driving faster than the tire's speed rating generates more flex and more heat.
At some point the tire can't dissipate the heat buildup. When that happens, the rubber fibers are damaged by the heat. Damage is said to be cumulative and irreversible. A little damage here, a little there, and eventually something gives.