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Friday, November 5, 2021

Why so much confusion on tire inflation vs Max Inflation vs my recommendation of +10%

A question posted on an RV forum

Tom said "So, I see some who are saying to set pressure to max cold pressure recommended, and others talk about “minimum +10%”…I’m confused."

Starting at the end.

Assuming you know the actual load on each tire from your measurement on a scale (ya I know about assuming but every RV owner has been told at least once to learn their actual loading).
You take the load on the heavy end of an axle as there are almost zero percent RVs with the load exactly at 50/50% side to side.

The load number is then found in the Load/Inflation charts for your size tire and you go up (to the right) till you find a block with at least or more load than what you measured on the scale. NEVER go lower than your scale reading. DO NOT average the reading from each end of the axle weight measurement. DO NOT try and calculate a pressure between the 5 psi increments.    Then look up in the chart to find the PSI.  That is the MINIMUM inflation you should ever run in the tires on that axle.

I suggest you add 10% to that inflation number to offer some "protection" in case the temperature drops. If you have added my recommended 10%, you will probably see that you do not have to add air every day the temperature drops 10 degrees.

RVs have Certification labels AKA Tire Placards that have tire size, type, Load range and inflation numbers. They also have GAWR which is the MAXIMUM load you should ever have on that axle.  The RV company is required by DOT, to post on the sticker, an inflation number that is sufficient to support 100% of the GAWR.  RVIA (a standards organization's sticker on the side of your RV now requires inflation level good enough to support 110% which is better than the DOT requirement)   Because of these load capabilities most RV companies select the smallest (lowest cost for them) tire that can just barely meed these requirements. The result of this purchasing decision is that you will need to inflate your tires to the level needed to support the tire's MAXIMUM load capacity which is the number on the sidewall of the tire.

Side issue. The wording on the tire sidewall is confusing. The reality of what it means is that any given tire has a MAXIMUM load capacity and an inflation (minimum) required to support that load. What is not printed on the tire sidewall is the fact that there is no increase in inflation that will result in that tire ever being capable of supporting more load. Therefore  the "max inflation" wording that was decided upon by some committee 50 years ago.


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