I understand how this incorrect information occurs when people assume “Max Pressure” means the maximum air pressure you can have in your tires. People decide to read the wording on the side of their tires and not the federally mandated vehicle certification label.
As I have said numerous times, “Max Press” on the tire sidewall IS NOT the highest air pressure an undamaged tire can tolerate. It is simply the highest pressure that will give you the highest load capacity. Increasing the pressure above that number WILL NOT give you any additional load capacity, since we all know, or should know, that it is the air pressure that supports the load and not the tire. Some think that they will get more load capacity by increasing the PSI, but that is not the case once you get to the “Max PSI” number on the tire sidewall.
Gross Axle Weight Rating (GAWR)
As far as tire loading and inflation goes, the MAX number is a hard stop, as is the GAWR (Gross Axle Weight Rating) for each axle. You can make modifications to the RV such as springs or air bags, or even tire size and Load Range, but those changes WILL NOT increase the GAWR.
Gross Vehicle Weight Rating (GVWR)
GVWR (Gross Vehicle Weight Rating) IS NOT the same as GAWR. It is the number for the heaviest the vehicle total can weigh. There are a number of RVs out there where the GVWR is lower that the mathematical total of the GAWRs. You should NEVER exceed either the GVWR or any of the individual GAWR numbers.
Cold inflation
Regarding cold inflation: This is the number found in the Load & Inflation tables and is the MINIMUM PSI you should ever run for a tire with a measured load. Running any lower pressure will “consume” the tire life and can result in tire failure. Damage to the tire structure is CUMULATIVE. Once you do some damage, no matter how minor, that damage will never go away and will never repair itself.
Damage occurs as you drive and can even occur when parked. Just because you are currently driving at 54 mph and at a load below the maximum shown in the tables does not mean that you cannot have a tire failure, as the tire might have been damaged by the pothole you hit 249 miles earlier.
Roger Marble
Check out my Blog www.RVTireSafety.Net
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Left home, at sea level, tire pressure at 75 psi. Now, at Tucson, TPMS indicates 85 psi. Both reading cold. Added no additional air. What’s up?
Altitude adjustment.
Altitude or Elevation can have an effect on tire PSI but not much as the effect of changes in temperature is greater. More info here https://www.rvtiresafety.net/search/label/Elevation
Regular tire pressure gauges don’t compensate for altitude changes. NASCAR uses $300(?) gauges.
I have no first hand knowledge on current NASCAR practices, but tire pressure is not normally adjusted for altitude changes unless you are “Driving” a 747 at 35,000 feet where the Absolute pressure is about 3.6. The pressure we are concerned about is PSIG (PSI Gauge) which is the difference between inside and outside the tire, not Absolute PSI. Absolute pressure is about 14.7 psi at sea level. Here is the Science. https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html
From what I understand the cheaper gauges lose accuracy at higher altitudes so when you drive back to sea level you will get the original reading.
Most likely was the change in Ambient temperature. TIRE PSI will change about 2% for every change in Ambient Temperature. Here are a couple posts on the Science of temperature and pressure. https://www.rvtiresafety.net/2021/11/adjusting-tire-pressure-because-its.html and here is the mathematical proof https://www.rvtiresafety.net/2014/03/why-does-my-tire-loose-pressure-simple.html
Seems like Roger just abruptly ended his article. Is the ending missing? Is there more?
Sorry. The post was just intended to clear up some confusion on the term used about tires. Background on each of these terms can be found in other posts on RVTravel.com Maintenence section or on my blog http://www.RVTireSafety.net
Thank you, Roger! 🙂 What an excellent overview; thank you! 🙂 Safe travels! 🙂