A recent thread on aftermarket TPMS systems reminded me to do something I have kept forgetting about.
Executive summary first, explanation (long) after;
Always adjust your tire pressures for temp if more than you like your pressures to be off of baseline.
Never adjust your tire pressures due to elevation change
We all have been around the perpetual discussions about tire pressure changes due to temperature changes, and they are quite accurate and predictable changes. On the other hand, pressure changes due to elevation change are often treated the same way, but probably should not be. Put on top of that the fact that TPMS systems don't all measure and/or display the tire pressures in the same units of pressure, and it gets even weirder.
The two different units used are the "gauge" pressure (psig) and the "absolute" pressure (psia). The difference is the starting point of the scale, with psia starting zero at a complete vacuum and psig starting zero at whatever atmospheric pressure is at the time and place measured. At sea level psig is zero and psig is 14.7psi.
If you already know about psig vs psia, jump to the end to see about the TPMS stuff.
The way psig is measured is how your hand tire gauges work. They just measure how much pressure you have above ambient atmosphere. To get psia you need to measure against a standard reference that never changes, which is much less practical in the real world and quite expensive to do.
How this all relates to tire pressures and elevation is not particularly instinctive, I think, and pretty misunderstood and often argued. Without going into the details of gas laws it comes down to that if no air is put in our let of fixed volume container, like a tire, the psia does not change with elevation. Psia will be the same at sea level as at 10,000 feet. Psig using exactly the same container and no air in or out will be about 4.6psi higher at 10,000 feet. The only difference is the gauge reference has changed for psig from 14.7 psi at sea level to about 10.1 psi at 10,000 feet. Basically psia minus atmospheric pressure equals psig.
For tires, all the air pressure does is support the weight on the tire by pushing back on the road though the contact area. The contact area part of the tire doesn't see atmospheric pressure so no allowance for any change needed. The psia in the tire hasn't changed at high elevation, so the contact area hasn't changed at all. This would all indicate that you should not reduce your tire pressure measured with a hand gauge when you go to high elevation, because the pressure that counts and is needed to support the vehicle hasn't changed for the elevation change. Mention this to many people and they will not believe it, as it would mean that the recommended tire pressure read on a gauge should be adjusted up for elevation increases.
Now put into the mix that most internally mounted TPMS sensors measure in psia as they have no external atmospheric reference unless it is done in the receiver, and it is better anyway as it is psia that counts. It also makes it so you don't get an overpressure alarm if you go up in elevation. It also makes it so if the temperature also changes a bunch, you will get an accurate underpressure alarm if it gets really cold at the elevation. Very confusing is that psig from going up in elevation can be just about equal to the psi (both psig and psia) lost because of the temp drop. You put your hand gauge on the tires that just gave a low pressure alarm and it looks just like it always has and where it was set at low elevation and high temp. You say it is a false alarm and take off on underinflated tires.
Now you take the case of the external sensors that may read in psig, like our TST, and it all changes as the TPMS will read just like your hand gauge. The TPMS will read higher pressure at high elevations, and you may get a high pressure alarm, that really is a error alarm as you should not change the tire pressures for elevation. Or, if the temp has fallen, you will not get any alarm so go merrily on your way when you should increase the pressure for the temp change, even if it sets off the high pressure alarm.
The ultimate confusion comes in something most of us won't see with class b's but is interesting, and I have actually been asked about this as it can get noticed. You have a tow vehicle with a factory tpms on it, and are pulling a trailer with a TST external sensors unit on it. You go up to 12,000 feet and the TST says the pressure is too high. You diligently let out air back until you hand held gauge reads what you thing is right (but isn't) and check the truck tires which also show high and you also air down then. Then the truck TPMS, which is looking at psia, trips a low pressure alarm which is actually correct as the tires are now actually too low. Almost nobody in the real world is going to know which is right, but nearly everyone in the real world will go with the two out three logic and have the hand gauge and TST overrule the factory TPMS. You go on your way with an alarm on the truck and underinflated tires on both the truck and trailer, but at least the TST quit beeping
There are a few disclaimers on this, though. If you can spare the capacity loss, if your tires are at max like most B van single rears are, it is questionable if you should reduce the pressure or not, as it is the casing you worry about. I posed this question to a couple of manufacturers and they said the amount they go above from elevation is small enough to not be and issue and overloading due to low psia support is very bad.
And, yes, the casing does grow a very small amount with the reduced atmospheric pressure on the outside, but it is a very small amount and the change in psia inside the tire does not change enough to put into the calculations, per the the manufacturers.
For me, if we are going to go through a high pass I will look at the temp difference we will see, and if it is more than about 30*F I will adjust pressure by about half of what the temp change would cause in pressure. That will cover the whole day without being more than a little over at the bottom and a little under at the top, no matter what the hand gauge and TST say.
I am relatively certain there will be lots of folks that don't agree with any of this, but that is fine. It would be interesting to hear any other information.