Tire PSI fluctuations

[timn1984]

I have a questions to throw out: My 2018 JLUS has about 4000 miles on it, and the tire fluctuation is between 4-6 PSI. So it came home from the dealership with about 38-40 PSI. I aired them down to about 34 PSI. throughout the day it may go up to 36 to 38psi. My question is should there be such a swing in tire pressure? I know it depends on the weather and it could be that I live in the Ohio Valley with great swings in temps in the spring and fall. I know there are many other discussions regarding tire PSI and I have read most of them, but am still puzzled as to what pressure is the most effective for every day driving. 30-32?? Should I air my tires down to 32 during the summer, knowing that they will rise to 34-36? Just want to get some feedback on this topic.

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[Jeepjunkie]

PV=nrT is Boyle’s ideal gas law.

As temp (T) rises (ambient temp or as the tires heats up from driving) pressure (P) increases accordingly, assuming the volume is the same (V).

To find the ideal tire pressure I do the chalk test (chalk line across the tire tread, drive 100-200’ & see if the line line disappears uniformly). My 285/70r17 KO2s needed 33-34 PSI when cold to have the chalk disappear uniformly. so I set my cold tire PSI a little lower (31-32) to allow for the tire temp and hence PSI to rise while driving.

 

[TRJeff]

You will normally see with street tires about 3% rise , if concerned try a chalk test, hot tires, chalk lines across face of tire s. Driving forward a short distance check lines for wear/ even use . Adjust accordingly

 

[timn1984]

PV=nrT is Boyle’s ideal gas law.

As temp (T) rises (ambient temp or as the tires heats up from driving) pressure (P) increases accordingly, assuming the volume is the same (V).

To find the ideal tire pressure I do the chalk test (chalk line across the tire tread, drive 100-200’ & see if the line line disappears uniformly). My 285/70r17 KO2s needed 33-34 PSI when cold to have the chalk disappear uniformly. so I set my cold tire PSI a little lower (31-32) to allow for the tire temp and hence PSI to rise while driving.

I didn't realize I was going to get a physics lesson, haven't had that since college days.......

 

[Jeepjunkie]

I didn't realize I was going to get a physics lesson, haven't had that since college days.......

It’s all good, that equation helps me remember that the tire pressure change is linear to the temp change. I certainly can’t use it for any real numbers calculation!

 

[BillG]

You are just worried because you have it on a screen right in front of you.
I was raised up to check tires before a trip, and if something looked low. I survived. Because it’s convenient now to check your tires, you’re noticing more variation.

 

[Chocolate Thunder]

The fluctuation you are having seems normal to me. Tire pressure increases about 1 psi for every 10 degrees Fahrenheit of ambient temperature increase. Or vice versa. So if you air them up to 34 psi when it’s 50 outside and the temperature increases to 70, they’ll be about 36 psi.

Also keep in mind that your tires get much hotter as you drive even when it’s cold outside, but much more so on hot pavement. This will account for several psi increase (depending on inflation pressure, pavement temperature, ambient temperature) while you drive. It takes a while to cool back down after you’ve stopped.

 

[RichD]

During the real cold days last winter, I was noticing a 1 to 2 degree difference from one side of the Jeep to the other. The reason, I realized, was long stretches on the highway with full sun on one side, the other cold shade. Yikes, too much information.

 

[Zazoh]

I’m seeing the same swing. I attributed it to more air and greater surface are in these tires than any other I owned.

 

[jeepdabest]

PV=nrT is Boyle’s ideal gas law.

As temp (T) rises (ambient temp or as the tires heats up from driving) pressure (P) increases accordingly, assuming the volume is the same (V).

To find the ideal tire pressure I do the chalk test (chalk line across the tire tread, drive 100-200’ & see if the line line disappears uniformly). My 285/70r17 KO2s needed 33-34 PSI when cold to have the chalk disappear uniformly. so I set my cold tire PSI a little lower (31-32) to allow for the tire temp and hence PSI to rise while driving.

For my Masters Thesis I wrote a paper about a little known law called...Fudds Law. Fudds Law=If you push something hard enough, it WILL fall over.

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