By Roger Marble
The question as asked is simple but, of course, I have to make the answer complex.
Not really. However, there are two different answers. One is for “motorized vehicles”: Class A, Class B, Class C and tow vehicles. The answer for trailers, be they teardrop or triple-axle 5th wheel trailers, has some minor but important differences. This post will address the motorized vehicles so I don’t have to keep switching back and forth.
Step 1: Learn the ACTUAL load on each end of each axle. This should be done when the motorhome or truck is loaded to the heaviest you would ever load it.
Step 2: Using the heavier end load number for each axle, consult the Load & Inflation tables for your size and load range tire, to learn the MINIMUM inflation required.
Step 3: Add 10% to the number in step 2. This is so you are not chasing inflation every time the ambient temperature changes.
Step 4: Set your LOW pressure warning level on your TPMS to the pressure in #2 minus 2 to 4 psi. (Note this variation is to account for gauge and TPMS pressure reading variation.) We want the warning to sound as soon as we might be overloading any tire.
Step 5: Using your hand gauge – that you have checked against your personal digital “Master Gauge” – set the “Cold Inflation” pressure for all tires on an axle to the same pressure, which would be the pressure in Step #3. NOTE “Cold” only means the tire has not been driven on or been in direct sunlight for the previous two hours. This means the tire is at “air temperature” or the “temperature in the shade.”
Step #6: Get in your vehicle and you can now drive.
Information: The +10% is to give you “wiggle room” for day-to-day changes in air temperature. We want to “protect” the “minimum” inflation number we learned in Step #2 while at the same time not have to get out and fiddle with tire pressure every morning. Each morning when you get up, you can turn on your TPMS and make your coffee. After a few minutes you can then read the TPMS monitor and as long as there is no unexpected pressure change (drop), you can be satisfied that all tires are sufficiently inflated.
You may see a slow drop in pressure over time even when there is no change in temperature. A drop of 1 to 2% per month is normal for all tires, but once you follow this guide you can probably expect to only need to add air once every two to three months. For folks needing High Pressure (80 to 130 psi) this means you can plan to “top off” your tires at the next truck stop, where they should have plenty of air at the pressure needed. I will cover adding pressure to a hot tire in a separate post.
Example: Following the above over my travels from Ohio to Oregon to Calgary to Glacier and home over a two- month period, I only needed to add air one time – at Yellowstone – and I only needed to add about 3-5 psi to my LR-E tires to get back to my +10% number in Step #3.
Comment: After a while you will learn that the inflation numbers in your tires are reported as slightly different than seen with your hand gauge. This is normal as most TPMS are rated at +/- 2% for pressure accuracy. The primary job of a TPMS is to report a pressure drop – not to report extremely accurate pressure. I would feel that if your hand gauge reads +/- 2 psi from a reference gauge that is good enough. If your gauge is off by 5% of your tire pressure goals you might want to get a better gauge.
Please do not forget the above is specific for “motorized vehicles” and not for trailers or dollies you pull. If you pull a dolly I might treat it more like a trailer with higher inflation than Minimum +10% – but that is a separate topic.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
 ##RVT968
My class A sticker says 70psi for cold. The tires themselves say 120 psi. Which number am I using?
As Roger mentioned, “consult the Load & Inflation tables for your size and load range tire, to learn the MINIMUM inflation required.”
The 120psi would be maximum for a stated maximum load of the tire, I suspect the tires installed are different than what is specified on the sticker.
Thank you Web.
Looks like you skipped over Step #1.
The 120 psi on the tires is what is needed if the actual load on the tire is the number of pounds identified on the tire near where you found the 120.
The 70 psi is what you would need if each tire on the axle was supporting exactly 50% of the GAWR number that is on the sticker AND you felt you need no Reserve Load.
Thank you Roger. I need to read more thoroughly next time.