Tire pressure. Sea level vs 5000 ft

[krhip]

So more that one tire store has told me tire pressure should be raised at altitude from the "sea level" tire pressure placard on the door.

[eseeling]

No. gauge pressure is gauge pressure (psig). The Absolute pressure (psia) of a system (your tire in this example) equals the atmospheric pressure (psi) plus the Gauge pressure (psig) In other words, psia=psi + psig. The absolute pressure of your tire may be lower at altitude, but the tire gauge pressure is relative to atmospheric. You are interested in gauge pressure regardless of altitude.
What causes problems is the TPMS which is calibrated for sea level (14.7 psi); therefore, the TPMS thinks the pressure is low. The atmospheric pressure in Parker is about 12.1 psi so the TPMS light probably won't go out until your gauge pressure is 2.5 psi more than "normal." that's true for my Subaru - I don't know about other TPMS's