Universal Tire Pressure Chart

[Route 66]

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

Why do you recommend that?

[Tireman9]

Route 66
As I pointed out in the post and as posted elseware all tires do lose air pressure over time. They also change pressure due to temperature ans altitude changes.
If you were to set your pressure at the exact minimum needed to carry your actual load then the temperature were to go down 10 or 20 degrees you would need to add a few psi air.
If you were to make a significant change in altitude you may need to add a pound or two to stay above the minimum.
If you were to experience a significant change in barometric pressure over a period of time along with the normal 1 - 2% per month you may also need to add a pound or two.

The reality is, you will soon get tired of adding one or two psi or you wil simply ignore the pressure because it becomes too much bother or you don't have access to air to inflate your tires.

So a +5 psi cushion would allow you to tolerate these day to day variations and you would probably never fall below the minimum for a number of weeks or possibly a couple of months. Eventually you will need to add air so you then can do so while at a location with an air hose.

+5 will not cause significant harder ride or unusual wear, especially considedring we are talking about your minimum being somewhere between 65 and 120 psi.

[Gary RVRoamer]

I too firmly believe in the "+5 psi" school of thought. I don't want to have to worry about being under-inflated. Besides, it is much better to be a little over a than a little under.

[Pairajays]

Tireman9

First off, I don't remember where I downloaded this chart, it was several years ago and was included in a writeup about tires and tire pressures. The entire article was quite lengthly and contained references to other articles which added validity.

Your qualifications certainly exceed mine when it comes to tires. I agree with just about everything you wrote, which by the way was quite informative but somewhat over my head. I wanted to make things simple and easy to understand.

First, you stated it was wrong to set the pressure at 93 lbs. if the temperature was 80 degrees. What should it be set at?

Next, where in my post did you see "there are statements and implications that appear to be misleading."?

Would you please reference a "Load & Inflation" chart from Michelin, Bridgestone or Goodyear that contradicts the subject chart? I can't locate one.

Another point of contention is the definition of "cold" inflation pressure.

And , finally, where did you get the 5# increase over the Load & Inflation chart?

Jim

[Gary RVRoamer]

I think the main point is that it is not necessary to continually adjust for ambient temperature. Just make sure the pressure is a bit above the minimum cold pressure for your actual load.

[Tireman9]

Originally Posted by Pairajays

Tireman9

First off, I don't remember where I downloaded this chart, it was several years ago and was included in a writeup about tires and tire pressures. The entire article was quite lengthly and contained references to other articles which added validity.

Your qualifications certainly exceed mine when it comes to tires. I agree with just about everything you wrote, which by the way was quite informative but somewhat over my head. I wanted to make things simple and easy to understand.

First, you stated it was wrong to set the pressure at 93 lbs. if the temperature was 80 degrees. What should it be set at?

Next, where in my post did you see "there are statements and implications that appear to be misleading."?

Would you please reference a "Load & Inflation" chart from Michelin, Bridgestone or Goodyear that contradicts the subject chart? I can't locate one.

Another point of contention is the definition of "cold" inflation pressure.

And , finally, where did you get the 5# increase over the Load & Inflation chart?

Jim

I would also like to make things easy. It really isn't difficult.
1. Get the real load on the tires
2. Look up or ask what the MINIMUM inflation should be for your personal conditions
3. Check the inflation with either a TPMS or every morning before you leave a campgrouund
4. To avoid going crazy don't worry about that last 1 psi. You only need to add air when you get down to the minimum so to make it easy just add 5psi whenever you inflate your tires.

Easy enough I think but some want to understand the why's behind this plan.

So lets see if I can answer your questions and not stir things up too much

The "cold" inflation definition is from The Tire & Rim Association Yearbook. This is the organization that tire companies and the US Dept of Transportation use when establishing tire dimensions, rim fit, loads & inflations. There are over 20 member tire companies. I can provide the names if you want but the bottom line is that if a tire company wants to sell aa tire in the USA the company must certify that each and every tire is capable of meeting certaiin tests. The loads, indlations and dimensions used to run those tests come from T&RA.

RE "Where do you find support for using cold inflation". Rather than bury the links in this thread You will find the links and more HERE.

RE the 93psi at 80 degrees. The correct inflation is set at ambient. That table may provide an approximation of what to expect in a change in tire pressure due to a change in temperature but should NOT be used as a guide for setting your tire inflation when the ambient changes. That table can incorrectly lead some to think they need to change their "cold" inflation based on some adjustment using the table.
That table is absolutly wrong if it leads some to incorrectly change the cold inflationnumber.

The +5 is only a "rule of thumb" intended to keep you from going krazzy adjusting your pressure every day.

Just stay above your minimum cold inflation before starting on any trip.

[Wayne M]

It all evolves to The Ideal Gas Law

Stop making me research all this information. I'm on overload.

It began with Boyl's law for me, as I studied in diving physics.

On edit: I check my tires every morning before departure. It takes only a few minutes of my time. If they are not to specifications I increase or decrease the air pressure. Coming out of the mountains to sea level I had to reduce pressure each morning. Once at sea level, and adjusted, they have remained constant at 110 psi.

[Tireman9]

Wayne
I offer all the links etc as some seem to want the backgound info, or even want to argue with published tire industry stanadards.

You appear to be following the simple 4 step plan outlined in my last post. Sounds good enough to me.
KISS is a good policy

[Flagelpater]

tireman9 Thank you very much for taking the time to answer all of our questions. I, for one, have successfully done as you have suggested, set 5 psi over the chart requirements for years. I use a TPMS and rarely need to make adjustments. I use this constant trend to indicate potential problems. Your explainations point me to the fact that there is a LOT more going on with our Tire System than simple temperature and pressure. Most of this is above my head but found to be very enlightening. My only remaining question is the requirement for different charts for different tire manufacturers. If this can all be reduced to the tables you use why do Goodyear and Michelin need to have different charts? Not argumentative here. Just wondering what I am missing. Also, could you elaberate on the contribution of tire pressure relative to the suspension system? Specifically, results of Over-inflation and shock absorption to the detriment of the structure of the suspension system. Is this even a problem?

[Tireman9]

Originally Posted by Flagelpater

tireman9 Thank you very much for taking the time to answer all of our questions. I, for one, have successfully done as you have suggested, set 5 psi over the chart requirements for years. I use a TPMS and rarely need to make adjustments. I use this constant trend to indicate potential problems. Your explainations point me to the fact that there is a LOT more going on with our Tire System than simple temperature and pressure. Most of this is above my head but found to be very enlightening. My only remaining question is the requirement for different charts for different tire manufacturers. If this can all be reduced to the tables you use why do Goodyear and Michelin need to have different charts? Not argumentative here. Just wondering what I am missing. Also, could you elaberate on the contribution of tire pressure relative to the suspension system? Specifically, results of Over-inflation and shock absorption to the detriment of the structure of the suspension system. Is this even a problem?

Flagelpater
Thanks. I would like to be able to provide info in a single line but tires are a lot more complex than 99.99% understand.

RE different tables. Again almost all manufacturers follow the same tables for almost all their tires. There are a very small number of exceptions.
These occur when one manufacturer is the only one making a specific size. Others will not bother since they don't make that size.
Same for some Load Range. i.e. Goodyear may make a LRH but others only make LRG.
I think I have read about one mfg having a single line of tires specifically for RV usage where they have adjusted the inflation by 5 or 10 psi but all else is the same.

So to keep it simple.
If you can find your specific tire listed by the manufacturer, then follow that table. Otherwise you can follow just about any table and be OK. If you see your size and Load Range listed in say Michelin and Bridgestone charts with the same load & inflation then you can probably follow those numbers for your "Mudwumper2" brand tire.

[Flagelpater]

Tireman9 Do you care to elaborate on my Over-inflation question? Also, again, Thank You for your very informative reply. This Forum is Great!

[Pairajays]

Tireman9,

So far no one has shown fault with the tire inflation chart that started this thread. Tire manufacturer's web sights have "Load & Inflation" charts but that is not what this chart is. It is a chart that shows what the inflation pressure should be at temperatures other than "cold", which is defined as 65 degrees F. So far that is the only place that defines what cold is except general descriptions such as, "first thing in morning", "after sitting 3 or 4 Hours", etc. I find this hard to believe.

Let's say you have Bridgestone, R280, 295/75R22.5 LRH on the front axel of your MH. One wheel weighs 6450# the other weighs 6500#. The Bridgestone load and inflation table indicates the minimum cold inflation pressure should be 105#, which will safely carry 6525#. (At this point, let's not concern ourselves with the 5# safety factor.) It is 6 AM, the middle of August and we are in Yuma AZ. The sun is not up and we have been parked for a week. The ambient temperature is 100 degrees F. We are going to leave for cooler temperatures in a couple of hours so we check our MH tire pressures. I say the pressure should be 113#.

Now, same situation except we are in Fargo ND and the temperature is 65 degrees F. I say the the pressure should be 105#.

What's wrong with this logic? It conforms to the chart in question and does not disagree with anything you have stated except it is wrong.

Jim

[Gary RVRoamer]

Nothing wrong with the chart or the logic. Whether it is necessary to use either chart or logic to insure safe operation of a motorhome is another question.

As for "cold", the nearest thing I can find to an authority is the National Highway Transport Safety Administration, which says:

The recommended tire inflation pressure that vehicle manufacturers provide reflects the proper psi when a tire is cold. The term cold does not relate to the outside temperature. Rather, a cold tire is one that has not been driven on for at least three hours. When you drive, your tires get warmer, causing the air pressure within them to increase. Therefore, to get an accurate tire pressure reading, you must measure tire pressure when the tires are cold or compensate for the extra pressure in warm tires.

Be your own judge.

http://www.nhtsa.gov/cars/rules/tire.../brochure.html

[Pairajays]

RV Roamer,

I think you are probably right and i agree with what you said. What you quoted from the NHTSA is aimed at people with cars that do not travel all over the country where large temperature changes occur within days instead of months. Also, a 4% or 5% change in pressure at 35# is not the same as that percentage change at 100#.

Jim