Is your RV tire temperature or pressure too high?

While I have answered versions of this question many times, people continue to ask this basic but very important question. Therefore, I will summarize the key information you need to consider in a series of posts. They will necessarily be longer than usual, and you may need to consult them more than once to ensure a proper understanding.

Some of this information comes directly from my RVTireSafety.net Blog, but it’s best not to just post a bunch of links. So here we go…

PART 1: How much air pressure is too much?

With the introduction of Tire Pressure Monitoring Systems (TPMS), many RV owners are now presented with new information about their tire pressure and temperature.

Let’s start with a common question: “I have a serious concern with the G-rated tires on my 5th wheel trailer. When I set the cold max pressure to 110 PSI, I get TPMS readings up to 134 PSI while driving. Lately, I’ve been running 95 PSI cold and getting 115-119 PSI while driving. The tires and rims are rated for 110 PSI. I can tolerate 120 PSI, but the 134 PSI concerns me.”

The inflation ratings for wheels are based on “cold” pressure. An increase in pressure due to operation is anticipated by wheel manufacturers, so I wouldn’t be concerned as long as the proper cold inflation is used, along with appropriate limits on load and speed. The term “Max Pressure” on a tire is often confusing. It actually refers to the cold pressure needed to support the stated “MAX load.” This load is the maximum weight the tire can safely carry.

I advised the owner not to underinflate the tires but to expect an increase in pressure due to operating temperature, as reported by the TPMS. The “cold” pressure is the only pressure you should worry about, and “cold” simply means when the tire is at ambient temperature—not warmed by driving or direct sunlight. Temperature in the shade is considered “cold,” even if it’s 45° F or 109° F. There is no “standard” cold temperature as we had in high school chemistry. Please disregard those “Temperature Adjusting Charts” for setting tire pressure.

Tire pressure changes with temperature

Tire pressure changes about 2% for each 10° F change in temperature. If you don’t remember this from high school science, you can read THIS post from my blog on Tire Safety. A 20% increase in pressure (110 to 132 PSI) indicates a 100° F increase in internal tire temperature. This is a bit high for normal tire operation; in my 50 years of experience, I only measured such a rise in Indy race car applications.

A 25% increase in pressure (95 to 119 PSI) means a 125° F rise in temperature, suggesting you’re pushing the tires very hard. This increased temperature accelerates tire aging, making the rubber less flexible, more brittle, and more prone to tearing. For more on how increased temperature shortens tire life, see THIS post.

Overloading tires and driving too fast

Increased temperature might indicate overloading your tires or driving faster than advisable for your tire load. Tire company data books recommend a maximum speed of 75 mph for RV applications. (I will address the 65 mph speed rating for ST-type tires later.) Speed ratings are short-term limits and should not be relied upon for day-to-day RV use.

The person asking the question should verify that their gauge reads 110 PSI accurately.

In response to their observation: “Today, I drove 250 miles with 95 PSI tires, which ran at 115-119 PSI. The tire temperatures were 20°F above the outside ambient temperature of 50° F, with tire readings at 71° F. I still believe that 134 PSI is dangerously high for 110-rated tires.”

Here’s my response: Few people realize that the pressure increase due to temperature is based on well-established physics. A TPMS doesn’t measure actual tire temperature but rather the temperature of the brass valve stem and the metal base of the TPMS, which is cooled by outside air.

Air is an excellent insulator. A small column of air inside the valve stem slows heat transfer. Also, the valve is rapidly cooled by outside air. Laboratory tests show that air inside a tire isn’t uniform in temperature but is cooler than the hot spots, which can cause tire failure if excessively hot for too long.

I’m confident that the TPMS reading of only 20° F above the 50° F outside air temperature is accurate. You should not worry about the hot pressure of 134 PSI if the cold pressure rating is 110 PSI, provided you confirm that the actual load on your tires is no greater than 90% of the load capacity shown in the Load & Inflation tables. By “confirmed,” I mean with actual scale readings for each axle when the RV is fully loaded.

As a tire design engineer with more than 50 years of experience, I trust the principles of the “Gas Law” and know that air is an insulator while metal conducts heat. I advise reducing the operating load and speed, and ensuring your gauge is accurate. Always inflate tires to 110 PSI when cold, as operating at current load and speeds will likely result in pressures well above the cold 110 PSI.

More on this topic next week.

Roger Marble

*****

If you have tire questions, check out Roger’s Blog as well as his posts on RVtravel.com. There are hundreds of posts covering everything to do with tires.

If you still have a question for Roger after searching the above posts, send your inquiries to him using the form below.

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