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TheJeeronian t1_ixrwfzy wrote

At 32 psi tire pressure (46 absolute) inside and 14 absolute outside, a decrease of 10% pressure would leave you with 41.4 absolute inside vs 12.6 outside. The relative pressure has dropped by, you guessed it, 10%. Now you're at a 28.8psi.

However, there are other factors at play, too. The atmosphere's pressure depends on both its temperature and volume, and its volume is not limited by any container, so a hotter atmosphere will expand but the pressure can stay the same.

In fact, the pressure does stay the same, because it is the weight of the air in the atmosphere that gives it pressure, whereas in a tire it is the tire walls stretching and keeping the air in that gives it pressure.

This all compounded by the nonlinear PV-T behavior of tire gas, especially water, which sees a huge pressure/volume drop as you approach 0 degrees c. Our atmosphere also has water in it, though, so without my first two points this wouldn't matter.

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xYoSoYx OP t1_ixrx84p wrote

Very good explanation…thank you!

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xYoSoYx OP t1_ixrxxlg wrote

Had another thought after rereading your last statement about water…

Would this mean that going from an extremely humid climate to an extremely dry climate would also impact the tire pressure? Just less significantly than a change in temperature?

Just curious if there is some mathematical equations that would represent the multiple factors you’ve laid out to explain the impact on the tires psi.

You’ve pretty much answered my initial question, just the math nerd in me wondering if this is a thing, so had to ask lol.

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TheJeeronian t1_ixs7fvp wrote

It shouldn't. As I mentioned, outside air pressure is pretty much fixed. There's a bit of variation but almost none. None big enough to be significant compared to tire pressure.

Humidity within the tire matters when it's inflated. A tire inflated with hot and humid air will lose a lot of pressure as it cools, but a tire inflated with cold and dry air will not gain nearly as much pressure as it heats up. This is because the amount of air and water in the tire is fixed, but the water may expand or contract quite a bit with temperature. The outside humidity once the tire is sealed makes no difference.

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xYoSoYx OP t1_ixs8lhk wrote

That makes total sense, again - thank you!

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