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Now a tire mostly have about a gauge pressure of a 30 psi, and the gauge pressure is the pressure above atmospheric pressure, and the atmospheric pressure is about 14.7 psi, does that mean that the gauge pressure is actually 15.3 psi? because instruments measuring gauge pressure takes atmospheric pressure as a starting point (0)

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  • $\begingroup$ Where did you get 15.3 psi from? Anyway, this isn't complicated. If you have a flat car tire, it still has air molecules in it but the pressure inside the tire is the same as the pressure outside the tire. That's why it's flat. That corresponds to zero gauge pressure for the tire. If you fill a tire to 30 psi (gauge pressure), then the pressure inside the tire is 30 psi higher than the pressure outside the tire. $\endgroup$ – Samuel Weir Feb 17 '18 at 19:10
  • $\begingroup$ 30-14.7=15.3, I think i'm missing up a bit.. $\endgroup$ – Yushi Feb 17 '18 at 20:15
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The gauge registers the difference between ambient pressure and tire pressure. A tire with 14.7 PSI absolute would read 0.0 PSI on a tire gauge at standard temperature and pressure (STP). Pressure would have to be 44.7 PSI ( 14.7 + 30) to read 30 PSI. Tom Brady can explain it to you.

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No. The gauge pressure outside the tire is 0. The absolute pressure inside the tire is 44.7 psi in your scenario. See this related question for details: Gauge pressure vs. absolute pressure?

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