In the Bay Area the last few mornings it has been very cold (35-38F). My TPMS warning came on and I checked all tires and they were all equal at 35 PSI (all were 40 when I checked about amonth ago).

Is about 5 PSI under when TPMS warns? Does it also warn at 5 PSI over?

I am guessing I should re-adjust for the cold weather and that it would also warn me if pressure is higher than the threshold...

 

The law only requires that you are warned when under pressure.

Also if you filled them to the proper pressure when 50F they could easily be over 5+ PSI higher after driving and the day warms up so warning for overpressure may not be effective.

 

Have your tires filled with nitrogen (at Costco or elsewhere) and you won't have this problem

 

...and most of the rest of it is Oxygen, which in "O2" form is also a "big" molecule that won't easily leak out of tires. The tiny bit of hydrogen and other smaller atoms or molecules in air just won't make much difference in pressure. Also, all gases behave the same way in response to temperature changes as far as pressure in the tire, so nitrogen has no advantage in maintaining pressure with temperature changes. (Remember the PV = nRT formula?) I've also read about a supposed advantage of nitrogen in that tires are supposed to last longer because there's no oxygen to deteriorate the rubber from the inside of the tire. Well, OK, over a few decades that might make some difference to the inside of a tire, but how do you eliminate the oxygen on the OUTSIDE of a tire?

Even if a person assumes that nitrogen all stays in a tire and everything else goes through the rubber or other small gaps (which isn't the case), as you keep adding more air to your tires to make up for lost pressure the percentage nitrogen would keep increasing inside the tire, well over 80% eventually. For example, assume you find the pressure a few psi low. At that point nitrogen is maybe at 90% in the tire because it didn't leak out. Then you add air and the nitrogen pressure ends up at 85%. Next time you are a few psi low and nitrogen is then at 92%. Then add air and you're at, say, 88% nitrogen...

If it's free, nitrogen is fine and even a tiny bit better, but the advantages are very small.

 

<"... and most of the rest of it is Oxygen, which in "O2" form is also a "big" molecule that won't easily leak out of tires ...">

it does leak out. measurably.
nitrogen is significantly more retained.

<"... The tiny bit of hydrogen and other smaller atoms or molecules in air just won't make much difference in pressure. Also, all gases behave the same way in response to temperature changes as far as pressure in the tire, so nitrogen has no advantage in maintaining pressure with temperature changes. (Remember the PV = nRT formula?) ...">

a normal fill's larger increase in pressure with heat is due to the amount of moisture in regular compressed air. the pressure change has nothing to do with the difference between nitrogen's expansion and oxygen's expansion.
the difference of nitrogen's lesser pressure increase with heat, is because a nitrogen fill is "dry".

this difference in pressure increase is not a huge benefit for most,
and even at the track, i have still found enough pressure increase with nitrogen, that i have had to fiddle with tire pressures anyway.

<"... as you keep adding more air to your tires to make up for lost pressure the percentage nitrogen would keep increasing inside the tire, well over 80% eventually. ...">

a proper nitrogen fill has to be at least 99%. any lower percentage negates the advantage.
the major drag about it is, you cannot "top it off" with normal compressed air, or you totally lose it's advantage, and have to go through the whole fill/purge/fill/purge/fill process all over again, to regain the 99%.
so, at the track you have to carry a nitrogen tank to up pressures.
in town, you have to go to a shop that has a nitrogen machine.

<"... If it's free, nitrogen is fine and even a tiny bit better, but the advantages are very small.">

some shops offer "lifetime" refills. that makes it worth it for some folks.
but i agree, the advantages are very small, and for most people the disadvantages in convenience far outweigh the advantage.

 

So how do tire shops get all the "air" out of a tire before adding their nitrogen fill? The one's I've watched don't. It's not possible to vacuum out the existing content, and I've never seen a shop purge the content out by repeated filling and then letting the gas back out (which would need to be done probably 5 or more times to be effective, including my recent set of Michelins installed at Costco. Therefore, there's 14.9 psi worth of 20%+ contaminants in there before adding nitrogen. Assuming the nitrogen is pure and the end pressure is 35 psi over atmospheric pressure, that means somewhere around 6% of the gas in the tire is not nitrogen.

Another problem with the nitrogen concept is that at some point everyone needs to add a little air at someplace where's there's no nitrogen fill available - gas station, home, etc. Personally, I find it makes much more sense just not to worry about it. Filling with air isn't a problem, and it has worked for generations. In other words, using nitrogen is fixing something that isn't broken.

I almost didn't post because there I was aware that people would attack the chemistry/physics, and say the ideal gas law doesn't apply to tires, in essence. I do have a degree and job in a chemistry field, and I'll leave it at that.

 

<"... It's not possible to vacuum out the existing content, and I've never seen a shop purge the content out by repeated filling and then letting the gas back out (which would need to be done probably 5 or more times to be effective, ...">

refilling and emptying 5 or more times is <b>exactly</b> the way they do it, until the percentage meter says 99

&lt;"... The one's I've watched don't, ..."&gt;

then they are doing it wrong, and ripping off their customers.

the reason a shop charges money for a nitrgen fill is for labour, not for the nitrogen.

 

Nitrogen is a gimmick.