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Old 05-25-2007, 07:47 AM   #1
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At home cold tire pressure was 95. Travelled to Florida, now the cold pressure is 100. I didnt think that air temperature effected tire pressures so im kind of confused. Perhaps my guage is the problem?

ps . Florida is nice in May if you dont mind a billion love bugs on your windshield.

Thanks

Steve
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Old 05-25-2007, 07:47 AM   #2
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At home cold tire pressure was 95. Travelled to Florida, now the cold pressure is 100. I didnt think that air temperature effected tire pressures so im kind of confused. Perhaps my guage is the problem?

ps . Florida is nice in May if you dont mind a billion love bugs on your windshield.

Thanks

Steve
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Old 05-25-2007, 08:20 AM   #3
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Mine does the same thing just sitting in the driveway. Varies about 5 to 10 psi depending on the outside temps.
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Old 05-25-2007, 09:25 AM   #4
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Steve,
I see nothing wrong with the difference in your readings. This happens to me every time I go up north. My tire pressure can be 5 lbs + different just because the sun hits one side of the coach and not the other side.
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Old 05-25-2007, 12:56 PM   #5
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I believe it is about a 1 psi fluctuation for every 10 degree F increase or decrease in ambient temperature.

We leave for Florida in late October and with our previous HR 95 lbs in the rear tires would increase (when cold) by about 5 psi a few days later in FL. Of course, it works the other way when we return so we don't bother adjusting.

Don
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Old 05-25-2007, 01:40 PM   #6
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Thanks for the replies.

I actually may have taken the original tire pressures in quite cool weather so this makes sense. I wont make any adjustments based on your good advice.

Thanks again.

Steve
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Old 05-25-2007, 02:38 PM   #7
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Steve, this topic can be "fun."

Earlier this week I got my monthly notice from GM's OnStar with diagnostics on my 07 Tahoe. This email provides current mileage, expected oil change mileage, engine and other status AND tire pressures (this is read from the sensors and passed on to OnStar via the cell phone built in.

Well, this week it showed one side was 1 psi over and the other 1 psi under recommended.

I went out to check it and it was just the opposite. The reason - they did the reading in the morning when the sun is one the tires on the left side. I went out in the afternoon when the sun is on the right side. I find the same thing with the MH. I have a Radio Shack infrared thermometer and when we stopped in our previous Ambassador I'd quickly check the temps to see how consistent there were. The sun side was always much warmer of course - for the outer dual - but not for the inner.

Don
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Old 05-25-2007, 06:12 PM   #8
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As a rule of thumb, tire pressures will increase/decrease about 2% (not psi) for every 10 degrees of temperature change. 2% change on your 95 psi amounts to about 2 psi for each 10 degrees.
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Old 05-26-2007, 08:15 AM   #9
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So if you start in a very warm climate at the suggested psi and travel to the extreme opposite cold climate (80 f down to 0 f) you could end up being 15 psi below recommended pressure by the time you arrive.
And here's a question,if you put the maximum 110 psi typed on the sidewall and then travel to the warmer climate and the psi increases to 125 are you now exceeding the tires max?

Steve
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Old 05-26-2007, 07:33 PM   #10
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When you read tire inflation guides they always refer to "ambient" temperatures. Why, because the outside temparature ALWAYS affects your cold tire pressure.

At the end of August, 2006 we left Pigeon Forge, TN for Essex Junction VT. We made a 2 week stop in Long Island, NY along the way.

In TN in warm summer temperatures I brought my tires up to 105 PSI (front) and 100 PSI (rear) after sitting for 4 months.

In NY where the temperatures were slightly lower my pressures read a lb or 2 lower.

In Essex Junction, VT in late September it was much cooler. My cold pressures were down by 5 lbs or so all around.

While attending the annual Newmar rally there I
sat in on the Michelin seminar. I asked an ambient air question: My pressures are lower here in VT, but after the rally I'm heading to Florida where it will be warm. Do I bring my pressures up now?

The answer was "yes & no." (great!) The Rep went on to explain that if you came from a warm climate to a cooler climate and then head back to warmer there is no real need to adjust pressures. If, however, you do bring the pressures up in a cooler climate, then when you get to the warmer climate you'll find that you have to bleed off some air.

I hop this makes some kind of sense to you.
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Old 05-26-2007, 07:50 PM   #11
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Not only temperature but altitude makes the air pressure change too.
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Old 05-27-2007, 02:05 PM   #12
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Generally you should not adjust pressures for either temperature or altitude unless you expect to stay in that area for some time. Now how much is "some"? Most alleged experts give answers like "several days" or "a couple weeks". The gist is that you do not need to adjust the tire pressure every morning, just because you drove a couple hundred miles to a slightly different climate, but you should adjust if the change is long term.

Probably the key thing to remember is, if you do make an adjustment, be sure to adjust back when you leave the area.
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Old 05-28-2007, 01:02 PM   #13
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In case someone is interested.
The formula for pressure increase in a constant volume container (tires are pretty close to a constant volume)is:

P1/T1= P2/T2

Where P1 is the initial pressure, T1 is the initial temperature.
Where P2 is the new pressure, T2 is the new temperature.
The temp must be in degrees kelvin.

Or P2 = P1 X T2/T1

For P1 = 85 psi, T1 = 294.26' kelvin, and T2 = 305.37' kelvin. (T1 = 70'F, T2 = 90'F)

P2 = 85 X 305.37/294.26 P2 = 85 X 1.038 = 88.21

So for a 20' F increase in temp the pressure should increase by about 3 psi.
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Old 05-28-2007, 01:39 PM   #14
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Way impressive formula. Way more work than I care to do. I appreciate your taking the time to let us know. But . . .

How about you just create a tiny little program that will let those who care to calculate the expected pressure change when taveling through different climates.

For me, I just don't care so long as my pressures don't drop below the minimum for my axle weights.

That's probably why I bought a Preseeure Pro last year.
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