Why wouldn't an N2 inflated tire change gauge reading with altitude? It's the outside pressure that changes when elevation changes, not the tire pressure.
Gauge pressure = pressure in vessel - ambient pressure. A tire inflated to 50 psi at sea level has 64.7 psi in absolute terms. If you go up to 14000 feet the ambient pressure will be 0.587 * 14.7 = 8.6 psi. Your gauge would then read about 56 psi. Nothing changed about the gas in the tire.
I'd guess the stress/strain on the tire carcass is a function of the differential pressure, so it makes sense to use gauge pressure where you are to inflate the tire.
Also, N2 definitely does change pressure with temperature changes. Pressure = gas constant * mass * temperature / volume to a first approximation for any gas at reasonable pressures. The constant may change but the proportion is there. Also, remember air is mostly N2 in the first place.
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