# Tire pressures

#### James Morse

##### 1997 XLT 4.0L 4x4 1999 Mazda B3000 2wd
Supporting Member
If I set my pressures at 31 psi @ 70F, when I measure them at 54 they are 29 psi. If I measure them with one side in the sun on a nice day, the sunny side is 33-34 psi (the other shady side, 31).

pv=nRT the gas laws. I'll leave it to someone else to figure out if my numbers above make sense - given they are just approximate - for example if air is 54F and it was a cold night tires could be colder.

In the gas laws equation for this purpose, volume doesn't change (unlikely the tire balloons/stretches out), only temp and pressure change in a well known relationship.

The point is, you can set your pressures whatever you like in the shop in known conditions, but to really know what they are it would be necessary to measure (and set, presumably) them just before actual use. For off road this wouldn't be a big deal because you could well be airing down/up so you'd do it within the immediate conditions. For normal driving, normally you don't check them all the time, however, I think we all can agree that if you're in a summer/winter climate and you set your tires good at 20F, when it's 80F it's not going to read the same and iirc we always let air out in the spring/summer and put air back in the winter.

A friend of my dad's, who we kind of made fun of at the time for doing it, without fail walked around his car before he got in to see, do the tires look good. Turns out this is actually smart, because if you just get in and start driving on a flat or low-pressure tire when you could have just fixed/aired it at home, you pay the price in various ways. It takes just a couple seconds to check both sides. That was in the 60's, and it seems like we had a lot more flats then, but it's still a good habit.

#### sgtsandman

##### Aircraft Fuel Tank Diver
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The general rule of thumb is that tires gain or loose about 1 psi for every 10 degrees of temperature change.

Tires in the sun and in the shade will complicate that greatly.

The best you can do is check your tires when there has been a big temperature change.

You also don’t want to do that within three hours of driving for more than about a mile because the tires warming up from driving will throw the psi off as well.

It can be a deep rabbit hole to dive into, if you let it, and will drive someone with OCD about it crazy. A couple psi off isn’t a huge deal. Once you get to 5 psi or more, I would worry and correct it.

#### mikkelstuff

##### Well-Known Member
U.S. Military - Veteran
I use this table generated for 35 psig tire pressure at 70 deg F and assuming a tire, near full, to be of constant volume.

#### rusty ol ranger

##### Im a Jeep guy now.
Supporting Member
You guys worry to much

#### RobbieD

##### 2.9l Mafia
You guys worry to much
You mean that you don't lay awake at night, wondering if the light in the refrigerator really DIDN'T turn off when you closed the door?

#### rusty ol ranger

##### Im a Jeep guy now.
Supporting Member
You mean that you don't lay awake at night, wondering if the light in the refrigerator really DIDN'T turn off when you closed the door?
Nope.

I do visually check tire pressure...or if its driving goofy...but generally i set em and forget em.

#### ericbphoto

##### Overlander in development
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#### ericbphoto

##### Overlander in development
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You mean that you don't lay awake at night, wondering if the light in the refrigerator really DIDN'T turn off when you closed the door?
No. It’s plugged into an AC source. It won’t run the battery down. A 40watt incandescent appliance bulb operating 24 hrs a day = 960 watt-hours or 0.96kilowatt-hours. .96kwh x 30 days = 28.6kwh per month. If I pay 10cents per kWh, that’s 2.86cents per month. But my refrigerator has an LED bulb that draws a small fraction of that. So, let it burn, baby! Let it burn!

Last edited:

#### RobbieD

##### 2.9l Mafia
So, let it burn, baby! Let it burn!
Y'all are missing the point- the turkey can't sleep with the light on.

Tired turkey is tough, and tastes terrible.

#### rusty ol ranger

##### Im a Jeep guy now.
Supporting Member
Y'all are missing the point- the turkey can't sleep with the light on.

Tired turkey is tough, and tastes terrible.
Turkey is always good

#### RobbieD

##### 2.9l Mafia
Turkey is always good
Until it's not. Let it sit in the fridge for about a month; green turkey that glows in the dark has stopped being a health food.

#### rusty ol ranger

##### Im a Jeep guy now.
Supporting Member
Until it's not. Let it sit in the fridge for about a month; green turkey that glows in the dark has stopped being a health food.
Scrape the maggots off and chow down. Dont be a sis

#### Uncle Gump

##### Token Old Guy
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I shoulda sent rusty that container of spaghetti I found today cleaning out the fridge.

#### gaz

##### Well-Known Member
U.S. Military - Veteran
@James Morse ,
I drive to the air station, air them all to 32. I verify the pressure with my cool-guy digital pressure gauge and call it a day.

After a 10 mile drive the tires are all nearly the same temperature, the air station is covered, never in the sun. It isn't perfect but it is as consistent as I can reasonably manage without making it a hobby or third job

#### rusty ol ranger

##### Im a Jeep guy now.
Supporting Member
I shoulda sent rusty that container of spaghetti I found today cleaning out the fridge.
You'd be surprised what ill eat.

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