There's an important point here: If you are setting 50psi with the tires cold, then their pressure at running temps could be 10psi higher than that. Tire pressures for street cars are usually specified as cold pressures, and allow for the increase in pressure due to normal highway running.
All that said, you may have been operating those tires at close to the max allowable pressure. In any case, running at pressures well north of 50psi for 40K miles will cause some deformation of the tread, and probably make the tires look cupped.
Its hard to imagine that anything over maybe 45psi hot temp would gain anything in MPG terms. Rolling resistance is important, but it quickly gets swamped by other factors when pushing a big truck through the air at highway speeds.
All that said, you may have been operating those tires at close to the max allowable pressure. In any case, running at pressures well north of 50psi for 40K miles will cause some deformation of the tread, and probably make the tires look cupped.
Its hard to imagine that anything over maybe 45psi hot temp would gain anything in MPG terms. Rolling resistance is important, but it quickly gets swamped by other factors when pushing a big truck through the air at highway speeds.