CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Tire pressure fluctuates - 2016 Ford Explorer

Can tire pressure go up and down without any air being put in them? My tire was down to 26, i put air into my tire and it was over inflated. I checked the automatic gauge and the other 3 were down also . A day later they went up a little then two days later the 3 regular inflated ones were normal and the under inflated one went back to an over inflated 37. ?

You don’t say where you are or what the current temperature is . Go to a tire store and have the tires set at the correct pressure , see what your dash read out says . Right now you are guessing and if you are in the desert a highway trip will cause the pressure to increase but not enough to worry about. You should also have a real tire gauge just to confirm what the dash readout says.

1 Like

Mine varies a bit due to outside temperature and whether the car has been sitting or I’ve been driving it and the tires are warm. The variation seems to go as much as 5 pounds. In the hottest part of the summer I was getting average readings of 38 lbs, and here in the dead of winter with the car not being moved I get 33 or 34 lbs. This seems normal to me and I’m not concerned.

1 Like

On a summer morning with one side facing the sun and the other in the shade, the sunny side tires will absorb a lot of heat and their PSI will get measurably higher.

1 Like

What is giving you the readings? The car or whatever you are filling the tires with?

I have taken three of those cheap 12V tire pumps with the gauges and compared them. Each was different. Maybe you are using something like this and it is reading low. So you pump up to the suggested tire pressure according to that device and the onboard reading is then too high. Remember that those cheap electric pumps may be great but are not always at the proper calibration.

I personally use one of these but you need to know the limitations. They are cheaply made and the accuracy of the built in gauge isn’t always great. I have one that pumps great but the gauge broke so have to use something else. This probably isn’t a bad thing as it was no good from the start.

Not sure about the tire pressure – that can vary w/the tire temperature and measurement variance – but it is very unlikely air molecules in any great number are moving from atmospheric pressure outside the tire to inside the tire at 26 + psi. I think that would violate some basic scientific law, but not sure which one … OP can get an idea of measurement variance by measuring the same tire 3 or 4 times in a row, maybe waiting 5 minutes between each measurement.

George,

First allow me to point out that this thread originated 4 months ago. I don’t know why I didn’t comment at the time, but it wasn’t obvious to the OP that the pressure in tires varies according to temperature.

But, there is the Law of Partial Pressure of Gases that says that a mixture of gases - as air is - should be treated as though the gases are individual. Usually we treat air as though it is one gas - and that usually works well enough - BUT - since gases will leak through a tire’s sidewall at different rates, we can’t do so in this case.

And to give you something to think about: If the tire starts with (more or less) 100% nitrogen, then the outside air’s oxygen will leak INTO the tire. I am not sure that would result in an increase in pressure, but it sounds like it might.

Note: On second thought: The rate that oxygen would leak through a tire is pretty close to the rate nitrogen would and since the partial pressures are such that there is a LOT of nitrogen pressure pushing out and only some partial pressure for oxygen pushing in, the pressure in the tire would be always be decreasing - except for the affect of temperature.

Good try…lol
But, at sea level the atmospheric pressure is 14.7 PSI, so the mixture of gases in the tire can only migrate out.
So we are dealing with a combination of kinetic theory, Newton’s third law, and Dalton’s law.
Physics is Fun—NOT

If the tire has 100% nitrogen, then a small amount of oxygen will diffuse into the tire, as @CapriRacer said. More than offset by the nitrogen diffusing out, and very very small. But it will happen.

The tire pressure will only go UP, if you have summer air in the tires during the winter season.

It will go DOWN, if you have winter air in the tires in the summer season.

So you have to have the correct type of air for each season. You also cannot mix the air types within one tire either…it must to be 100% the correct type for the season at hand.

If you find my answer plausible, then I have a very exclusive business proposition for you. Its a bridge purchase, smack dab in the middle of NYC !! This is a very exclusive proposition and guaranteed to earn huge profits instantly. Please do not inform any others about this offer, its a cash deal only however, but huge returns on your investment.

2 Likes

Count me in on the bridge :wink:

@Abdul-Wakil are you using the same gage when you check the pressure, there can be variations between them, especially the inexpensive ones. Do not rely on the gage at a filling station pump, they are not known for accuracy and may not be repeatable (not giving the same reading over and over). If you do not have your own gage head on over to an auto parts store and get a good one.

The person started this in September , 2018 with 1 post and has not returned with any more information as to how they check the pressure or even their location. It makes it kind of difficult to even offer guesses.

Plus the chance summer air will freeze and crack your tire or rim.

2 Likes

Readily available here:

http://kalecoauto.com/index.php?main_page=product_info&cPath=9&products_id=31&zenid=pU2pzYb7v1QO5AbFjWZeT0

2 Likes

They have some very hard to find parts. I misplaced my round tuit, they have them!

Here’s one you can use

a-round-tuit