Tire Pressure Cold vs Hot

[ErylFlynn]

So we had a big trip and I set the pressure to 37 per the door panel. While driving I monitored it and found that rear tires seemed to increase a lot more than the front. Was running hot at 41 front and 43 rear. I assume this is normal, is this a problem? Should I lower the rear tires a bit?

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[kevman65]

Cold, set them at 32 psi. They will get to 36-38 psi when warmed up.

 

[TheSolarWizard]

I set my cold pressure based off a chalk test and if the payload is maxed I add 3 to the rear.

Driving around though the pressures seem to vary wheel to wheel based off freeway speed and which tires are getting more sun

 

[smlobx]

If you’re running the stock tire size use the cold tire pressure as stated on your door placard. Don’t worry about how hot it gets. That’s what you paid the Jeep engineers to figure out.

Hot tire pressure is ONLY an issue in race cars where you want the tire to be at a specific narrow range (across the entire width) when up to operating temps, usually in a lap or two (former Porsche racer).

 

[Empty Pockets]

If you are worried about it now, wait until the seasons change. In GA it is not uncommon to go from 32 in the morning to 37 in the afternoon

 

[PW45]

the PSI listed on your door sticker is COLD psi so yes because of Pv=nRt the temperature and pressure will increase as you drive but I wouldn't worry about that.

 

[Mr._Bill]

I have the stock street tires on 20" wheels. I fill them to 39psi cold. They will go as high as 45psi traveling at 75mph in the summer heat. I run them a little high to try and help with fuel economy. They came from the dealer with a Nitrogen fill. I don't know if that makes any difference, other than not needing to add air as often.

 

[MrBones]

They came from the dealer with a Nitrogen fill. I don't know if that makes any difference, other than not needing to add air as often.

Mostly smoke & mirrors sales gimmick. Lots of studies determined loss of pressure diference and better fuel economy are minimal. My JTM came with green valve stem caps so I assume they used nitrogen but I'll fire up my HD compresssor when they need air and I won't lose any sleep over it.

 

[KevinC]

Mostly smoke & mirrors sales gimmick. Lots of studies determined loss of pressure diference and better fuel economy are minimal. My JTM came with green valve stem caps so I assume they used nitrogen but I'll fire up my HD compresssor when they need air and I won't lose any sleep over it.

I mostly agreed with your statement for many years. My wifes 22 JLU came with nitrogen filled tires. I lowered them to 33 psi from the 45 the dealership had them at during the PDI. I have yet to add any air in 2-1/2 years of ownership.
OTOH - my JTM with tires filled with regular O2, I am constantly adding air when the weather changes. If I air them back up +-15psi after an off road trip with my portable CO2 tank, it will fluctuate even more.

I wouldn't pay for Nitrogen, but it does seem to hold air better than O2 or CO2.

 

[ASU_Greg]

For every 10*, you will either gain/ lose 1 PSI.

 

[JTL21]

I mostly agreed with your statement for many years. My wifes 22 JLU came with nitrogen filled tires. I lowered them to 33 psi from the 45 the dealership had them at during the PDI. I have yet to add any air in 2-1/2 years of ownership.
OTOH - my JTM with tires filled with regular O2, I am constantly adding air when the weather changes. If I air them back up +-15psi after an off road trip with my portable CO2 tank, it will fluctuate even more.

I wouldn't pay for Nitrogen, but it does seem to hold air better than O2 or CO2.

Here’s the thing with nitrogen fill…
Nitrogen is a larger molecule than oxygen or carbon dioxide so there is something to the theory that it won’t escape as easily but we’re talking about molecular level measurements. The other thing to remember is that our atmosphere is predominantly nitrogen to begin with…around 78% by volume. So what people are paying for is the other 22% and when you think about it in those terms it starts to get hard to believe there’s much benefit.
There’s more benefit to the person using old beat up steel wheels because nitrogen is a noble gas and won’t cause steel wheels to rust like regular atmosphere air can. Most of us use aluminum wheels so it doesn’t really matter.
Chances are your wife’s situation is a coincidence and it’s more likely they got a little lucky with a really good seal when they mounted those tires.

 

[Rusty PW]

So we had a big trip and I set the pressure to 37 per the door panel. While driving I monitored it and found that rear tires seemed to increase a lot more than the front. Was running hot at 41 front and 43 rear. I assume this is normal, is this a problem? Should I lower the rear tires a bit?

The increase on the rear is normal.

 

[Blade1668]

I regularly see a deviation of a few psi in air pressure between the tires. And will second the that nitrogen fill is mostly a snake oil sales except for aircraft and race cars as 99. 9% vs 78.09 nitrogen. To verify that I and JTL21 would not be giving bad data I did quick check. Especially since it's been close to 30 years since I did my scuba open water course.

 

[Vtur]

The more air a tire can hold, the higher deviation between cold vs hot. Higher elevation also raise pressure too.

 

[Gatorized]

I mostly agreed with your statement for many years. My wifes 22 JLU came with nitrogen filled tires. I lowered them to 33 psi from the 45 the dealership had them at during the PDI. I have yet to add any air in 2-1/2 years of ownership.
OTOH - my JTM with tires filled with regular O2, I am constantly adding air when the weather changes. If I air them back up +-15psi after an off road trip with my portable CO2 tank, it will fluctuate even more.

I wouldn't pay for Nitrogen, but it does seem to hold air better than O2 or CO2.

Here’s the thing with nitrogen fill…
Nitrogen is a larger molecule than oxygen or carbon dioxide so there is something to the theory that it won’t escape as easily but we’re talking about molecular level measurements. The other thing to remember is that our atmosphere is predominantly nitrogen to begin with…around 78% by volume. So what people are paying for is the other 22% and when you think about it in those terms it starts to get hard to believe there’s much benefit.
There’s more benefit to the person using old beat up steel wheels because nitrogen is a noble gas and won’t cause steel wheels to rust like regular atmosphere air can. Most of us use aluminum wheels so it doesn’t really matter.
Chances are your wife’s situation is a coincidence and it’s more likely they got a little lucky with a really good seal when they mounted those tires.

since nitrogen is the larger molecule, it must be the o2 leaking out, which means the more that leaks, the higher ratio of nitrogen remains in the tire…. After multiple refills and leakage, You should eventually end up with just nitrogen in your tire for free!

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