Thanks, gaijinnv. That forced me to research the issue of water vapor - and it turns out that water vapor DOES act like an ideal gas, but not near the saturation point - which is where the water precipitates out (rains!), and that is sometimes the condition of the air inside a tire.
But if we are all to get on the same page, I think we have to agree on some things:
- Even if the inflation medium inside a tire starts at 100% dry, it will not remain so: The Law of Partial Pressure of Gases
- Even though there may be water inside a tire at the time of mounting, that water will not remain liquid water over the long haul. Again, the Law of Partial Pressure of Gases.
- Regardless of the inflation medium. the inflation pressure in a tire will rise due to heat - both ambient temperature change and heat buildup due to operation. The Ideal Gas Law
- Any potential difference in pressure buildup is only significant in race tires, both because of how small that difference is and because the pressure buildup hardly matters. In race tires, that pressure buildup affects the spring rate of the tire and therefore how the car handles. Every where else, whatever the water is doing doesn't affect the operation of the tire. Even as cold as aircraft tires get, the amount of water vapor precipitating out would be very small.
