# Tire Pressure
**URL:** https://boards.straightdope.com/t/tire-pressure/708500
**Category:** Factual Questions
**Created:** [December 30, 2014, 2:51am UTC](https://boards.straightdope.com/t/tire-pressure/708500 "2014-12-30T02:51:07Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@glennwith2ns](https://boards.straightdope.com/u/glennwith2ns)
#### Post date: [December 30, 2014, 2:51am UTC](https://boards.straightdope.com/t/tire-pressure/708500/1 "2014-12-30T02:51:07Z")
If inflate an unmounted tire to 40 lbs , mount the tire on a car and then lower the jack will it remain at 40 lbs ?
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### Author:  [@chacoguy](https://boards.straightdope.com/u/chacoguy)
#### Post date: [December 30, 2014, 3:08am UTC](https://boards.straightdope.com/t/tire-pressure/708500/2 "2014-12-30T03:08:03Z")
Unless it leaks, there;s no way for air to get in or out.
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### Author:  [@Morgenstern](https://boards.straightdope.com/u/Morgenstern)
#### Post date: [December 30, 2014, 3:17am UTC](https://boards.straightdope.com/t/tire-pressure/708500/3 "2014-12-30T03:17:37Z")
Seems it would go up slightly as the weight of the car compresses the bottom of the tire. Just a guess, but it’s an easy concept to test.
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### Author:  [@Isilder](https://boards.straightdope.com/u/Isilder)
#### Post date: [December 30, 2014, 3:23am UTC](https://boards.straightdope.com/t/tire-pressure/708500/4 "2014-12-30T03:23:47Z")
> [@glennwith2ns](#):
>
> If inflate an unmounted tire to 40 lbs , mount the tire on a car and then lower the jack will it remain at 40 lbs ?
Pressure is more normally stated in in psi … pounds per square inch…
The question as I understand it, was does the pressure stay constant ?
No. But it only changes a tiny amount, it would be hard to measure it.
The flat spot is only a small length compared to the circumference, and the change in volume is then only tiny. The higher pressure stretches the circumference out some tiny tiny bit, which adds back some of the volume lost to the flat patch at the bottom, and so the change in pressure is reduced… it can never be zero or negative, but its tiny.
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### Author:  [@Xema](https://boards.straightdope.com/u/Xema)
#### Post date: [December 30, 2014, 4:19am UTC](https://boards.straightdope.com/t/tire-pressure/708500/5 "2014-12-30T04:19:40Z")
Yes, the change is very small - probably too small to measure.
A small amount of Googling led to [this discussion](http://www.internationalskeptics.com/forums/showthread.php?t=198796), which includes this useful post:
> [@ben m](#):
>
> Say you’ve got a 15" diameter, 6" wide, 35psi tire supporting 1/4 of a 2600 lb car. That requires, in principle, an 18 sq.in. contact patch, so 6" wide and 3" long. So, how much do you have to move in from the edge of a 15"d circle to find a 3" chord? Only about 0.15". The area outside this chord is about 0.17% of the area of the disk. Likewise, the volume “displaced” in crushing a 15" tire from round to a round-with-a-3"-contact-patch, is 0.17% the initial volume, so via PV = nRT, for a small perturbation, it’ll increase the pressure by about 0.17% or 0.06 psi.
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### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [December 30, 2014, 4:49am UTC](https://boards.straightdope.com/t/tire-pressure/708500/6 "2014-12-30T04:49:49Z")
> [@glennwith2ns](#):
>
> If inflate an unmounted tire to 40 lbs , mount the tire on a car and then lower the jack will it remain at 40 lbs ?
Unmounted can have two meanings. First is the tire is not mounted on the rim. second the tire/wheel assembly is not mounted on the car. I have to admit when I first read the OP, my thought was How the fuck do you get 40 PSI in a tire that is not mounted (on a wheel)?
:dubious:
I can tell you from experience, there is no easily measured difference between airing the tires up off the ground and on the ground (except is is usually easier off the ground)
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [December 30, 2014, 11:46am UTC](https://boards.straightdope.com/t/tire-pressure/708500/7 "2014-12-30T11:46:16Z")
Similar thread: [Does tire patch area + tire air pressure = car weight?](http://boards.straightdope.com/sdmb/showthread.php?t=160240)
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### Author:  [@Xema](https://boards.straightdope.com/u/Xema)
#### Post date: [December 30, 2014, 3:11pm UTC](https://boards.straightdope.com/t/tire-pressure/708500/8 "2014-12-30T15:11:54Z")
> [@Crafter\_Man](#):
>
> Similar thread: [Does tire patch area + tire air pressure = car weight?](http://boards.straightdope.com/sdmb/showthread.php?t=160240)
Not one post in that thread pointing out that ‘+’ in the title should be ‘\*’ or ‘x’ ? Dopers have clearly refined their nitpickiness in the intervening 11 years.
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### Author:  [@Fubaya](https://boards.straightdope.com/u/Fubaya)
#### Post date: [December 31, 2014, 1:55am UTC](https://boards.straightdope.com/t/tire-pressure/708500/9 "2014-12-31T01:55:30Z")
> [@Xema](#):
>
> Not one post in that thread pointing out that ‘+’ in the title should be ‘\*’ or ‘x’ ? Dopers have clearly refined their nitpickiness in the intervening 11 years.
It’s closer to 12 years.
Nitpickiness restored.
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### Author:  [@Nefario](https://boards.straightdope.com/u/Nefario)
#### Post date: [December 31, 2014, 12:37pm UTC](https://boards.straightdope.com/t/tire-pressure/708500/10 "2014-12-31T12:37:45Z")
> [@glennwith2ns](#):
>
> If inflate an unmounted tire to 40 lbs , mount the tire on a car and then lower the jack will it remain at 40 lbs ?
For practical purposes - yes.
If you had thick, tough tires that could barely deform, it might be _slightly_ higher.
You lose credibility by equating weight to pressure so does the answer matter? :rolleyes: