Let's assume TPMS is calibrated @ sea level (which I'm pretty sure it is), and you have a super-accurate handheld digital gauge. Target pressure is 42 PSI all-around.
If you fill your tires to 42 at sea level, everything is in-sync: TPMS reads 42, and so does your handheld gauge. FYI - handheld gauges measure pressure relative to ambient pressure, TPMS measures in absolute since it's in a closed system (the tire). Which is why you shouldn't trust TPMS for absolute measurements.
Anyway, let's then assume you drive up 5,000 feet in altitude, where the atmospheric pressure drops. You look at your TPMS screen, and it reads 44 all-around, because of the lower ambient pressure.
If you now take a reading with your gauge at this new +5,000 ft altitude, I believe it would also match the TPMS reading @ 44???
However, if you adjust your tire pressure at this new +5000 ft altitude back down to 42 using your handheld gauge, your TPMS would read lower - say ~40 PSI? If my understanding here is correct, this is where I'm getting confused... why is it a 1:1 on your way up, but not 1:1 once you adjust your pressures to the correct PSI at your new +5,000 ft home? Unless the gauge would initially read even higher than the TPMS reading prior to adjustment, say 46... in which case this would all make sense.
If you fill your tires to 42 at sea level, everything is in-sync: TPMS reads 42, and so does your handheld gauge. FYI - handheld gauges measure pressure relative to ambient pressure, TPMS measures in absolute since it's in a closed system (the tire). Which is why you shouldn't trust TPMS for absolute measurements.
Anyway, let's then assume you drive up 5,000 feet in altitude, where the atmospheric pressure drops. You look at your TPMS screen, and it reads 44 all-around, because of the lower ambient pressure.
If you now take a reading with your gauge at this new +5,000 ft altitude, I believe it would also match the TPMS reading @ 44???
However, if you adjust your tire pressure at this new +5000 ft altitude back down to 42 using your handheld gauge, your TPMS would read lower - say ~40 PSI? If my understanding here is correct, this is where I'm getting confused... why is it a 1:1 on your way up, but not 1:1 once you adjust your pressures to the correct PSI at your new +5,000 ft home? Unless the gauge would initially read even higher than the TPMS reading prior to adjustment, say 46... in which case this would all make sense.