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Old 07-21-2010, 04:18 PM   #1
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Universal Tire Pressure Chart

Here is a good tire pressure chart that will cover all situations. Be sure and pay attention to the notes. It defines "cold pressure" which is correct any time of the year anyplace in the country.
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File Type: pdf Equivalent Tire Inflation Values.pdf (10.2 KB, 6245 views)
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Old 07-21-2010, 05:16 PM   #2
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Wow - what the chart suggests seems counterintuitive to me. Logic (well, MY logic, anyway) tells me a fixed charge of air in a closed container (in this case a tire) will increase when ambient temperature increases and decrease when ambient temperature decreases.

So . . . IF one was trying to compensate for ambient temperature changes when airing up a tire (in practice, I just check "cold" for 65 psi, no matter the ambient), seems the proper thing to do would be opposite of the chart.

BUT, I'm open-minded --- I might be wrong.

What is the source of the chart?
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Old 07-21-2010, 05:49 PM   #3
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Yeah, me too. It's going to take some convincing for me to get my head around this being a good idea. At first I thought I agreed with it but it would have me adding air to my tires before starting out across the desert in Phoenix and I'm just not going to do that.

I too would like to know the source of the chart.
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Old 07-21-2010, 08:00 PM   #4
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Quote:
Logic (well, MY logic, anyway) tells me a fixed charge of air in a closed container (in this case a tire) will increase when ambient temperature increases and decrease when ambient temperature decreases.
Your logic is correct and that's what the chart says. If you had set tire pressure at 75 psi at 65 degrees, then at 75 degrees that same tire would have increased to 77 psi. Therefore if you checked the pressure at 75 degrees ambient, you should expect 77 rather than 75 psi. That's why you don't let air out just because it is a bit warmer outside.

You still have to get the correct pressure from the tire manufacturers inflation chart, but recognize that those pressures are given for a "cold" tire, meaning 65 degree ambient temperature. If you fill the tire with air at 85 degrees ambient but use the "cold" 65 degree psi, you are probably under-inflated by something like 4%.
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Old 07-21-2010, 10:36 PM   #5
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Quote:
Originally Posted by SeeTheUSA View Post

So . . . IF one was trying to compensate for ambient temperature changes when airing up a tire (in practice, I just check "cold" for 65 psi, no matter the ambient), seems the proper thing to do would be opposite of the chart.
the point is that we shouldn't need to compensate... and we don't.
not for the ordinary driver in ordinary conditions.

even broad changes in temps and altitudes during the course of a given day shouldn't require external compensation.

extended stays at different conditions than whatever your baseline may be also shouldn't require a different set of numbers to be used... just a consistent approach to using them for check/fill (eg early am, cold etc)

so if you go from running in the hot desert to 6000 foot mountain range for a few days do check your tires the next morning and if needed adjust them back to your base psi numbers.
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Old 07-21-2010, 11:57 PM   #6
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RV Roamer,

You got it right. This chart is good for any tire from any manufacturer. Just remember manufacturer's cold tire pressure is at 65 degrees F. Cold does not mean "not driven for several hours", "after sitting over night", etc. Note the left column "Recommended Cold Inflation Pressure" is the same as the blue column, ie, 65 degrees F. In a perfect world, once set, you would never have to add or let out air regardless of the outside temperature. One exception. If temperature drops below 65 degrees, you must add air to bring the tire back to the 65 degree pressure.

Jim
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Old 07-22-2010, 11:18 AM   #7
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Whew - one less thing to worry about!

Thanks.
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Old 07-23-2010, 02:47 AM   #8
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Thanks for sharing Pairajays and thanks for the explanation Gary. I think I've gotten my thick skull to let this sink in and since I spend a good bit of time in the desert this is actually important to me.
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Old 07-23-2010, 06:40 AM   #9
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try airing up in the winter in New England before heading to Florida. Then you'll find out real quick what temperature rise is all about after you it the road.!!
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Old 07-23-2010, 08:31 AM   #10
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Your elevation factors into tire pressures as well. Here is a good article from Tire Tech Information - The Influence of Altitude Changes on Tire Pressure

The Influence of Altitude Changes on Tire Pressure

Significant changes in altitude affects tire pressures when traveling from one elevation to another. Fortunately this influence is relatively small and can be easily accommodated. Atmospheric pressure is the force exerted on objects by the weight of the air molecules above them. While air molecules are invisible, they have mass and occupy space. However as altitude increases, atmospheric pressure decreases. For example, atmospheric pressure pushes against the earth at 14.7 pounds per square inch (1 kilogram per square centimeter) at sea level, yet drops to only 10.1 pounds per square inch at 10,000 feet as indicated in the following chart:



When it comes to measuring tire inflation pressure, it is important to realize there is a difference between atmospheric pressure and gauge pressure. Most pressure gauges (including all tire pressure gauges) are designed to measure the amount of pressure above the ambient atmospheric pressure.

Imagine removing the core from a tire valve and allowing the air to escape. Even after the air has completely stopped rushing out of the valve, the tire is still experiencing 14.7 pounds per square inch of atmospheric pressure. However, a tire pressure gauge would read zero pounds per square inch of tire inflation pressure because the pressure outside the tire is equal to the pressure inside. Since a tire mounted on a wheel essentially establishes a flexible airtight (at least in the short term) pressure chamber in which the tire is shaped and reinforced by internal cords, it retains the same volume of air molecules regardless of its elevation above sea level. However, if tire inflation were set with a tire pressure gauge at sea level (where the atmospheric pressure of 14.7 pounds per square inch is used as ambient atmospheric pressure by the gauge), the same tire pressure gauge would indicate the pressure has increased at higher elevations where the ambient atmospheric pressure is lower. Those measured at the 5,000-foot level (where an atmospheric pressure of only 12.2 pounds per square inch is the ambient pressure) would indicate about 2-3 psi higher than at sea level. On the other hand, traveling from a high altitude location to sea level would result in an apparent loss of pressure of about 2-3 psi. However, the differences indicated above assume that the tire pressures are measured at the same ambient temperatures. Since tire pressures change about 1 psi for every 10-degree Fahrenheit change in ambient temperature, the tire pressure measured in the relatively moderate climate typically experienced at sea level will change when exposed to the colder temperatures associated with higher elevations. This means that in many cases differences in ambient temperature may come close to offsetting the differences due to the change in altitude. Depending on the length of their stay at different altitudes, drivers may want to simply set their cold tire pressures the morning after arriving at their destination, as well as reset them the morning after they return home.
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Old 07-23-2010, 11:18 AM   #11
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Wow - what the chart suggests seems counterintuitive to me. Logic (well, MY logic, anyway) tells me a fixed charge of air in a closed container (in this case a tire) will increase when ambient temperature increases and decrease when ambient temperature decreases.
I can see where you are coming from but.. You forget a law The law is Charles's law - Wikipedia, the free encyclopedia on the link.

As a gas is heated, in a closed container, two things happen

One the container expands... Slightly, But the other is the gas tries to expand as well, Far Far more than the container,, THUS the pressure rises.

This is called Charles' law and .. The link above explains it

This was high school physics when I was a child. Likely elementary school now if "Smarter than a 5th grader" is any indication of how things have changed. I took college physics so....

The bottom line is as temps rise, so does pressure.

Have you ever watched a car fire.. With very few exceptions, and most of them in Hollywood, a car will not explode in a great ball of flame (Perhaps it he's transporting a case or two of things filled with gunpowder)

But there are smaller explosions, 4 or 5 of them tops.. The tires.

A combination of the hot rubber being softer and the pressure rising till BANG. the tire blows.
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Old 07-23-2010, 09:33 PM   #12
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Since tire pressures change about 1 psi for every 10-degree Fahrenheit change in ambient temperature,
That's not universally true. The pressure changes about 1.8% for every 10 degrees of temperature, so an RV tire that normally runs at 100 psi will show a nearly 2 lb difference. A car tire at 30 psi, on the other hand, will only change about 0.6 lbs per 10 degrees.
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Old 07-23-2010, 10:50 PM   #13
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Man, I love this forum.

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Old 07-24-2010, 11:02 AM   #14
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Smile Some info from a retired tire engineer

Pairajays
Could you please provide the source of your chart "Equivalent Tire...."
While the numbers are probably mathimatically correct for the theoretical pressure change due to temperature change if the inflation gas is dry air, there are statements and implications that appear to be misleading.

According to The Tire and Rim Association, Cold Inflation in Load and Inflation tables is "The inflation pressure taken with the tire at prevailing atmospheric temperatures and do not include any inflation build-up due to vehicle operation."

Tire & Rim Association publishes the standards for dimensions and loads that are supported by over 20 different tire manufacturing companies in the US and around the world. The loads and inflations as published are the basis for the test requirements to be deemed as in compliance with Federal regulations as established by the US Dept. of Transportation. If your tire is sold in the US for use on public highways it is supposed to carry the DOT symbol. If your tire has the DOT symbol it can be interpreted as meaning that the tire manufacturer certifies that that tire when new was capable of passiing any of the required strength, temperature or durability tests.

Based on some comments in this thread some are incorrectly assuming that if for example they look on the Load & Inflation chart for their tire and see that based on their actual measured load need at least 90psi but the ambient temperature is 80 degrees then they need to inflate their tires to 93psi.

THIS IS NOT CORRECT.

Some are providing examples of setting their inflation in a cool area like Flagstaf then driving to a hot area like Phoenix where there could be a change in ambient of 30 to 50 degrees. They forget that in general, properly loaded and inflated tires will develop internal temperatures of 20 to 60 degrees above ambient (depending on type, load & speed etc).
The way to avoid problems is to check your inflation in the morning before you start a day's driving and before the tires are heated by a few hours exposure to direct sunlight. Set them to no less than the minimum required to carry the actual load based on what is shown in the tables.

Now before you start to worry about needing to add air every couple of days as temperature and altitude change it is a reasonable rule of thumb to follow this practice;
1. When fully loaded (that means full gas,water,food, clothes, pets and co-pilot etc) get your RV weighed so you know the real load on each corner.
2. If significantly (more than a few%) unbalanced side to side try ane re-pack the heavy stuff.
3. Consult the Load & Inflation charts as published by your tire manufacturer. If you can't find them then I would look at the charts from any of the major companies such as Bridgestone, Goodyear or Michelin.
4. In the table:
a. Find your size (don't confuse an LT245/70R17 LR-E with a 245/70R17 LR-G
b. find the box that has the load that exceeds the heavier side load for that axle.
c. Look up to find the inflation required for that load
5. I would suggest you increase your inflation by 5 psi as long as you are not exceeding the MAX stated on your tire or the MAX on your wheel
6. All tires on an axle should carry the same inflation so you do all this based on the heavy side
7. Don't forget to use the correct load for a dual application.

If you have followed the above you will probably be able to go for weeks without needing to add air to stay above your minimum and when you do add air you just go to the +5 psi inflation.

Finally if you are having difficulty in getting the above information and can't find a dealer that understands this then PM me and I will try and help out.
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