In my search for articles covering this topic, the only clear articles I have ever found on this topic contained the usual instruction to measure and set the pressure when the tires are "cold" (without saying exactly what that means), or after letting them cool for at least a couple of hours in the shade before measuring and setting the pressure. Other articles I have found are aimed at truckers or bicyclists, or are unclear how to apply them, or are too inaccurate for my taste. For example, a few say the rule of thumb is to adjust 1 PSI for every 10°F your tires are warmer than the cold-tire temperature. However, as you will see below, that figure is correct for only a very small range of riding conditions, and not conditions we usually encounter.
You would think that tire manufacturers as well as auto manufacturers would have clear instructions on how to correctly deal with hot tires, but they do not—at least not that this author has seen.
From these facts, I get the feeling that authors that have the know-how to write about this topic are not willing to touch this subject for fear of being held liable for an accident. Indeed, both tire manufacturers and auto makers only publish advised tire pressures when the tires are "cold". I am guessing this is because their attorneys told them not to elaborate beyond that.
So I will make this disclaimer right here: the below information is neither a recommendation nor advice. Rather, it is an academic discussion of the physical properties of tires with respect to temperature and pressure.
To begin our academic exploration, I will disclose that I work with physics daily as part of my job, and that I have tested the below empirically with my Tire-Pressure Management System (TPMS) as well as 2 expensive tire-pressure gauges that match each other and match the TPMS (so I have assumed they are accurate) and find that, in my laboratory of the great outdoors, the results appear to be consistent with the theory.
