Air pressure at seal level is 14. 7 psi. At 18,000 feet it is approximately half of that, or 7. 35 psi. That would mean that at the relatively low altitudes at which roads exist that the pressure differential couldn't exceed about 7 psi maximum. In my case, I live at about 4100 feet elevation. If pressures were set here, they wouldn't deviate by more than about 3. 5 psi lower at sea level and 3. 5 psi higher to over 8000 feet elevation. I think that's basically meaningless for all practical purposes regarding automotive tires and probably explains why the tires sidewalls don't give specific instructions for altitude - pressure compensation.