Tire Pressure Vs. Ambient

[bluwtr49]

I recently found that adjusting our tires to the cold inflation temperature base on the local ambient may not be the correct way to do this. Based on this info, the ambient temp for setting tire pressure is always 65F and they are than adjusted upwards base on local ambient.

This was a surprise to me and would suggest that many are running their tires under inflated.

Tire Pressue vs Ambient

Something to consider. Me, I'm going to follow this guide.

[rgatijnet1]

Tire Rack air pressure tests and results.

It seems they are not concerned about 60 degrees, but the "cold" tire air pressure before you start driving for the day, whatever the temperature is.

[Ohio_Engineer]

two-niner wrote:
JetAonly is correct. Everything is based on 59 degrees F. at sea level.

Except for the fact that the Tire industry Load & Inflation tables are based on tires being at 'Ambient" temperature.

The use of the word "cold" has mislead some to start thinking about the need to refrigerate their tires.

Tire Engineers are almost exclusively worried about excess heat so we may think of Ambient as "cold" being the opposite of hot.

Too many make this much more complicated than it needs to be.

- Know the real load on your tires
- Know the minimum inflation pressure needed to carry the load
- Add a bit for a margin (10% is reasonable as long as you don't exceed the wheel or valve rating)
- Confirm your gauge is reasonably accurate i.e. +/- 2 psi or better from a new digital gauge or gauge at tire store.
- Run a TPMS
- Enjoy your RV

The above works at any elevation you will be driving your RV. It will also work for Ambient of -10F to 130F. Outside this range other factors may come into play so you would want to contact the tech folks at your tire company to review your situation

[Ohio_Engineer]

rgatijnet1 wrote:
When I check my tires before getting on the road, I am looking for 95 psi, no matter what the outside temperature is. 95 psi is 5 psi over what the manufacturer's chart recommends as the minimum based on what my coach weight dictates.
If it is less, I add air until my tire gauge reads 95 psi and do not concern myself with the outside temperature.

x2 :B

[two-niner]

JetAonly is correct. Everything is based on 59 degrees F. at sea level.

[JetAonly]

My motorcycle reports tire pressure adjusted to 60F, FWIW.

Most of us don't have an air pressure gauge that is accurate within .5 psi. Further, tires don't generally fail because they are 2-3 psi over or under. If we are going to obsess over a couple of psi for temp. then you'd better start checking ambient altitude and adjust at 1psi/2000ft.

IMHO

[RCMAN46]

I have a TPMS system.

I have found all 10 of my sensors read within 2 psi of each other.

I set my pressures in early morning inside my shop building. As such all tires will be at the same temperature.

I then check the pressures with the TPMS. I try to get all pressures the same for each axle.

Now the most important thing is that all pressures on each axle is the same each morning before the sun hits any of the tires.

Then after on the road for 10 miles or more all pressures for tires on each axle should be (and I have found it to be true) the same within 2 psi.

If I see any pressures that are different by more than 2 psi compared to my early morning check then I find a safe place to get off the highway and try to determine why I have a pressure difference.

With the TPMS no need to use a gauge except when setting initial pressures. The TPMS may not be totally accurate but I have found they are very consistent from day to day.

[holstein13]

Ductape wrote:
+ 1 to JumboJet answer of the year!

This discussion also cannot be complete without discussing elevation. As you rise, air pressure lessens, so correspondingly Psig in the tire increases. Meanwhile, you are further from the center of the Earth, so gravity is diminishing. Do we need a correction table for altitude? I think pilots use them.

Two things:
1) as you rise, temperature drops so tire pressure increase would be offset by the temperature decrease effect. and
2) That's kinda true / trueish. Even if we floated 30,000 feet above sea level, you'd weight .3% less than at sea level. But .3% isn't a lot and we never drive that high so I don't think we need to worry about that. Besides, we typically drive on roads which are supported by mountains which increase gravity because they have more mass. So it's probably closer to zero effect on gravity.

It's also interesting to note (but almost completely irrelevant to this discussion) that your tire pressure gauge displays PSI relative to ambient pressure so 95 PSI is 95 PSI above the outside ambient pressure.

On edit: to further quantify the effects of atmospheric pressure, the air pressure at sea level is about 14.7 PSI. In Denver, approximately one mile high in elevation, it's 12.2. So I guess your tire pressure will read about 2.5 PSI higher in Denver vs the starting elevation if you started at sea level. Even if you drive up to 10,000 feet, the atmospheric pressure will be 10.1 PSI so you'll have 3.6 more PSI. All this, of course, assumes the same temperature tires, but the outside air will be cooling your tires the higher you go.

[Kaz]

OK, I got an answer from Roger Marble. Note that the short answer is that one should just add 10% to the minimum cold inflation pressure for the loading on that axle, which avoids the necessity of "chasing your tail" on the temperature-pressure curve. Hope this helps.

Skip

-----Response-----

"The concept of "Standard Temperature" comes from laboratory experiments "Since temperature and air pressure vary from place to place it is necessary with a standard reference condition to compare testing and documentation of chemical and physical processes."
Tire inflation guides are not a chem lab experiment guide but are guides for proper inflation needed to carry a specific load in the real world. Tire manufacturers do not say to set tire pressure to "Standard" conditions but to set the pressure when the tire is at "Ambient" temperature i.e. not artificially increased due to running or exposure to a direct heat source. The term "cold inflation pressure" should be synonymous with being at ambient temperature and not warmed up from running. Note it also does not mean you have to place your tires in a refrigerator to get them "cold"

RE the need to "correct" the cold pressure. Since cold pressure means at ambient there is no correction needed.
While it is always possible for some to come up with travel plans such as leaving Pike's Peak at -10°F and driving to Death Valley where it is 120°F that night so they want to know if they should set the pressure for some future ambient temperature and elevation. I would suggest that this is not necessary as even this extreme change would only result in a theoretical pressure increase of about 14 psi which the tire can tolerate. If you still feel the need to compensate then take a 2 hour lunch and adjust your pressures at that time.
For more normal travel I suggest simply adjusting your "cold" inflation when you haven't driven on them for two to three hours and they are not in direct sunlight. Morning before travel usually meets these guidelines.
I will fall back to earlier posts and info on my blog where I tried to summarize inflation guidelines.
1. Using your actual tire loads know the minimum inflation needed when the tires are at ambient temperature by consulting tire mfg Load/Infl charts. Lets call this the Minimum Cold Inflation Pressure or MCIP.
2. All tires on an axle should be run at the same inflation so this means you use the heaviest loaded tire to learn the MCIP for the tires on that axle.
3. Always run your tires at or above the MCIP.
4. To avoid chasing your tail when the temperature drops which means you need to add 1 or 2 psi I suggest adding 10% to MCIP and as long as you are within the range of 100%MCIP to 110% MCIP all is OK.
Hope this helps.
Roger

[Ductape]

+ 1 to JumboJet answer of the year!

This discussion also cannot be complete without discussing elevation. As you rise, air pressure lessens, so correspondingly Psig in the tire increases. Meanwhile, you are further from the center of the Earth, so gravity is diminishing. Do we need a correction table for altitude? I think pilots use them.

[holstein13]

rgatijnet1 wrote:

holstein13 wrote:

rgatijnet1 wrote:

In my case, since I checked my tires when cold, before I got on the road, (This may not be a convenient thing to do based upon your location or time of day.) if I noticed a tire that was LOW at the first rest stop, I would know I have a problem with that tire. (I'm not sure how you would "know" you have a problem. I think you would ASSUME you have a problem. But I certainly would not ASSUME I had a problem. Maybe some kid just let some air out.)I may put air in it to get to a service center, but I certainly would not ASSUME that the tire was now safe and would remain safe/perfect the next day.(Sounds to me like you are making other assumptions. Not me. I'll inspect the tire and keep an eye on the tire pressure monitor and I'll figure out using facts whether or not I have a problem.) If it was a dual, I would also be concerned about the other dual that had to take up the additional weight to compensate for the tire that was low.

Is this the same monitor that did not warn you the tire was losing air until you noticed it at the rest stop? :B

Not me. This one's built into the coach. The last one I had, I finally gave away because of the problems with it. I had a problem with this one losing connectivity but they fixed it. I can always see when it's working. It's very accurate and displays both the pressure and temperature.

[JumboJet]

Groover wrote:

pconroy328 wrote:
Never mind the fact that most of use pressure gauges with absolutely no idea of their accuracy or precision. While the physics may be sound, PV = nRT, it strikes me as needless hand-wringing and worrying.

"Gee, it's 72* outside, let me check the conversion chart... OK, now I'll pull out my $3 pressure gauge and check..."

I agree. A lot of people are overthinking this and placing too much confidence on a gauge of unknown accuracy. Just get a good gauge, check it against a few others and then check you pressure before driving. Maybe run a few PSI higher than the minimum recommended to allow for gauge error, slow leaks, load changes, etc. Check your gauge against others periodically to be sure that something hasn't happened to it.

Another THUMBS UP!

I check my gauges against at least two others. I purchased a digital gauge a few weeks ago and was amazed at the accuracy of my analog gauge compared to the digital.

[bluwtr49]

I look at this chart as one more tool that is available to help make good decisions. The whole tire pressure "cold" has been a big moving target when traveling through different climes. This puts that moving target into perspective by assigning a reference attribute.

Saturday I will check tire pressure in anticipation of a Sunday departure to head North. The ambient temp will be at least 90, perhaps more once I get out to the barn. With a normal cold inflation pressure of 90 psig I expect the tire to be at 96 or so. Conventional wisdom would suggest that I drop the pressures back to 90 but that runs counter intuitive to me since I know the ambient will drop dramatically by Tuesday when I hit Elko and even more when we arrive Idaho Falls. For the time being, I'm going to adjust the pressures using this chart and than keep an eye on how it goes.

I think one of the values of this info is that it gives a guide as to what your pressures should be at different temps. If you have a good feel for that, abnormalities would become more evident. My TPMS readouts are very accurate so it's even easier to check every morning while traveling.

Of course, as stated, the tires are never run below your carrying capacity pressure regardless of temp.

[Groover]

pconroy328 wrote:
Never mind the fact that most of use pressure gauges with absolutely no idea of their accuracy or precision. While the physics may be sound, PV = nRT, it strikes me as needless hand-wringing and worrying.

"Gee, it's 72* outside, let me check the conversion chart... OK, now I'll pull out my $3 pressure gauge and check..."

I agree. A lot of people are overthinking this and placing too much confidence on a gauge of unknown accuracy. Just get a good gauge, check it against a few others and then check you pressure before driving. Maybe run a few PSI higher than the minimum recommended to allow for gauge error, slow leaks, load changes, etc. Check your gauge against others periodically to be sure that something hasn't happened to it.

[rgatijnet1]

When I check my tires before getting on the road, I am looking for 95 psi, no matter what the outside temperature is. 95 psi is 5 psi over what the manufacturer's chart recommends as the minimum based on what my coach weight dictates.
If it is less, I add air until my tire gauge reads 95 psi and do not concern myself with the outside temperature.