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Kind of dumb question - leaving car sitting and tires deflate ...

iwannam3

Member
Aug 8, 2016
905
1,273
Washington
May help to have 100% nitrogen in the tires. That was the rage 15 years ago - big marketing push with car dealers.
"Air" is 78% Nitrogen. Did they take all the "air" out of the tire before they added the nitrogen? If you want 100% N2 you have to evacuate the tire before filling it. Let's do the maths. Start with 78%, raising the pressure by one bar (14.7 psi) and you double the amount of gases in the tire so now it is 11% oxygen, add another bar (29.4 psi now) so you have 3 times the amount of gasses so O2 is now 7.3% but it is still the same number of O2 molecules.
 

cwerdna

Active Member
Jul 11, 2012
3,443
2,292
SF Bay Area, CA
I checked them with a good tire gauge (warm and cold) -- and the TPMS readings were correct. I guess I've never had a car sit barely used this long to see this occur. Is this normal and to be expected?

I used a tire pump and brought them back to 44 psi (warm). They seem to hold the pressure fine.
As already has been stated, it's normal.

As for checking tires "warm" vs. "cold", you should be checking them when they are cold (when car has sat overnight or not been driven for MANY hours) as driving them heats up the tires and raises their pressure. You should inflate to the recommended cold pressures when the tires are cold.

If you do it when the tires have already heated up due to driving, you'll have to make your best guess or take pressures before driving and make a best guess as to how many psi to add/bring it up to.
 

WarpedOne

Supreme Premier
Aug 17, 2006
4,346
6,393
Slovenia, Europe
"Air" is 78% Nitrogen. Did they take all the "air" out of the tire before they added the nitrogen? If you want 100% N2 you have to evacuate the tire before filling it. Let's do the maths. Start with 78%, raising the pressure by one bar (14.7 psi) and you double the amount of gases in the tire so now it is 11% oxygen, add another bar (29.4 psi now) so you have 3 times the amount of gasses so O2 is now 7.3% but it is still the same number of O2 molecules.

OT:
The point of filling the tires with nitrogen is not in filling it with nitrogen :)
The point is in not adding additional water vapor that is present in outside air.
Cheapest easily and widely available non poisonous compressed gas happens to be nitrogen. It contains almost no water.

Problem with water vapor is it condenses/evaporates, drastically changing volume and thus pressure.
This problem arises only when your wheels get hot enough (boiling temps) after you filled the tires on cold and humid day.
A non-problem for civil driving.
 

SageBrush

REJECT Fascism
May 7, 2015
12,243
15,163
New Mexico
so you have 3 times the amount of gasses so O2 is now 7.3% but it is still the same number of O2 molecules
... as you had when the tyre was deflated, which is about 1/3 the # of O2 molecules before the deflation. I infer then that pressure loss will be ~ 1/3 the rate seen when tyres are inflated with ambient air.

Which brings us back to the N2 marketing gimmick. IIRC, it was devised as a loyalty ploy. When done 'correctly' the store would inform the car owner that with each subsequent top-off with N2, the pressure loss rate would decrease. Once the consumer started down the N2 path, the store owner hoped that the car would be brought to the store every couple of months.

Clever, in a pathetic kind of way
 

one2many

Member
Aug 16, 2019
572
744
germany
Sorry I just woke up, I use N2O for my car. I tell you its a laugh
a minute even if does not go into the tires. If N2 is better and O2 is good,
then N2O must be the best. If you ever get stuck on the side of the road,
you can always make whip cream, it is sure better with beer.
Smooth ride.
 

Uncle Paul

Well-Known Member
Nov 1, 2013
6,253
6,768
Canyon Lake,CA
Except for racing cars, which generate very high tire temps, Nitrogen is simply a marketing gimmick.
Kind of like putting colorful NASCAR style manufacturers stickers on the outside of your tires. More style than function.

Some mention that they rarely topped off their cars tires from years ago. This is because they did not have federally mandated tire monitors going off on their dashes. They often drove around with underinflated tires because they were uninformed.

Another issues is that EVs typically have much higher inflation pressures to make them more efficient and add range. This higher initial pressure also will cause them to loose a few pounds quicker as well.

For the most part, we see lots of tire pressure posts at this time of year because temperatures are dropping.
 
Oct 28, 2019
93
64
Ridgewood NJ
It's the temperature - does anyone remember chemistry (pressure divided by temperature is a constant value). When the tires increase temperature (when driving), the pressure goes up. Sitting in the cold garage, you will (temporarily) lose pressure.
Sorry to be nerdy about this, but I'm a (somewhat recovered) engineer.
 

M3BlueGeorgia

Active Member
Dec 10, 2018
1,349
1,077
Atlanta, GA
It's the temperature - does anyone remember chemistry (pressure divided by temperature is a constant value). When the tires increase temperature (when driving), the pressure goes up. Sitting in the cold garage, you will (temporarily) lose pressure.
Sorry to be nerdy about this, but I'm a (somewhat recovered) engineer.

In the Fall, I add about 4 lbs of pressure to each tire, then in the Spring I remove the extra pressure.

My objective is to ensure my tires have 46-49 lbs of pressure after being driven on the Freeway for half an hour.
 
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KenC

Active Member
Sep 4, 2018
3,490
3,176
Maine
For those who didn't get the joke, the little white ball, aka widget, inside a can of Guinness®, is actually filled with nitrogen, to create a smoother foam.
 

one2many

Member
Aug 16, 2019
572
744
germany
It's the temperature - does anyone remember chemistry

Yes I went to the university (whats a mater U) Whats your favorite color, what is the air speed of a swallow.

After repeating the beer can suggestion, my tires are fine, but I have a little excess gas, maybe
the beans for lunch, thanks for asking.

Happy hour calls again in Japan I think.
 
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PDB2655

Supporting Member
Sep 30, 2020
53
24
Stuart,Fl
Tires we always have to keep an eye on our tire pressures the air leaks out from are rims due to expansion and contractions or hot or cold this is normal. I check mine and add maybe monthly or when needed I adjust to to 44 or 45 psi some can go years others days or months. Its a good idea or even a good habit to check your pressure regularly adjust when needed this maximize your tire life add better mileage and is generally a great thing to do.
 
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jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
8,208
9,058
Riverside Co. CA
So, I have to say, threads like this are one reason I got into visiting this site every day, and spending so much time here reading random threads.

On every other car board i have been a member of, this question normally comes up around the end of fall, when the temperature changes, with some variation of a complaint around tire pressure. Normally, on most car forums, the post would be met by a couple of posts that say "yep its normal, put some more air in it when it gets cold".

HERE, however, in addition to that, like clockwork, people usually start talking about the composition of air molecules vs nitrogen, the dynamics of what actually causes the pressure loss, and other "geeky / sciencey" stuff. Tesla attracts technical people, for sure. :D
 

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