The gauges at the gas station measure the overpressure in the tire, i.e., how much higher the pressure in the tire is than the current ambient air pressure. (See: Wikipedia - Tire pressure) However, ambient air pressure depends on a location (altitude above sea level) and the atmospheric temperature, and on average decreases by approximately 100 hPa (equivalent to 0.1 bar) for every 1000 meters of altitude. ...
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