I know I have answered this question in the past, but maybe it was on an RV forum for a single-brand RV so not everyone has heard this. So here goes.
Many, but not all, tires have the words “Max psi” followed by a number on the tire sidewall. I have no idea which lawyer wrote the requirement, but he or she didn’t understand how those words would be misinterpreted.
Too often I see people saying that the tire should never be run with a higher pressure than that stated on the sidewall, or that this is the absolute only correct pressure for the tire. But these assumptions are incorrect.
Tire load capacity is related directly to tire inflation
I hope everyone reading this post understands that tire load capacity is related directly to tire inflation. If they want to increase the load capacity, they will need to increase the inflation. While it is true that an increase in tire inflation is required if you want more load capacity, there is a limit. The limit is controlled by industry standards which are published and followed by all tire companies.
Each tire has a Maximum Load Capacity. To get to that capacity you need to increase the tire inflation. BUT there is a limit, as each tire also has a limit or maximum load capacity. Increasing the inflation above the stated pressure WILL NOT increase the load capacity.
So you have a tire that says “Max 65 psi”—which means you will gain increased load capacity as you increase inflation from 35 to 45 to 60 and to 65 psi. BUT any additional pressure above 65 psi WILL NOT RESULT in any additional increase in load capacity.
It is also important to know that tires are tested and can tolerate higher pressures due to being warmed up by running. The pressure on the tire sidewall only refers to the tire “cold” inflation, so you should not bleed down the hot pressure.
Have a tire question? Ask Roger on his RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
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Roger, occasionally while driving through the desert SW, my TPM will send me an audible alert that I am over pressure. When I start out that day, I may be 95 psi and the alert hits at 123 or 124. Generally it is the Inside tire on the curbside of the bus. Should I worry?
Tires will gain pressure as they heat up and the reason inflation pressures need to be checked when cold. Anytime there is something noticed that may or may not be an issue, it deserves further investigation. An inside tire might be exposed to higher heat from the engine or an exhaust component. Or it might be that the tires, even if the same size, but are different brands, could be mismatched in diameter. The larger diameter tire will carry more load and run somewhat hotter. Also, measuring the pressure of the other tires on the axle with a gauge is important too. If all the tires are near the same pressure, then the one tire pressure sensor might be a bit inaccurate since variations can occur between sensors. Also, look at the maximum sidewall pressure. If pressure increase is not that much over maximum sidewall pressure, it is normal heating due to friction and payment temperature. Imagine if the tire were inflated to maximum sidewall pressure cold, how much higher the PSI might go.
Thank you. I appreciate the time you took to reply to my question.
Also, what is the minimum pressure required to avoid excessive and uneven wear?
With belted tires, a wide flat fabric or steel belt that runs the diameter of the tire under the tread, normally the lowest pressure to avoid uneven wear is somewhat below the maximum tire pressure as the belts tend to keep the tread flat and wearing evenly even if somewhat over or under inflated. Tire pressure is normally decreased for ride comfort. Lower pressure will allow for more sidewall flex and adsorb some of what the suspension does not. However, it is important not to allow inflation pressures to fall too low or risk a condition known as a “Run Flat.” A run flat is a tire were the tire cords have become damaged by under inflation and can only be detected by dismounting and inspecting the inside of the tire and its liner. On truck and bus tires, a loss of 20-percent normal tire pressure is considered a run flat.
I used to ‘fret’ when I noticed how much higher my tire pressures would get as we traveled down the road. Especially in summer temps and elevation. Now I adapted the old song about “Don’t worry, be happy”.
With bias ply tires there was more to fret about. With cooler running radial tires, yes, not as much fretting as they tend not to gain as much pressure in operation.
So, Roger, to me the question is why wouldn’t you run the cold tire pressure at the max pressure for max load capacity when towing, for example, a 5th wheel? At least on the truck’s rear tires?
The answer depends on the load capacity and size of the tires. From your question, I see that you have a Pickup. Without actual data (tire size, LR, and measured load on your rear axle) I can only guess and I don’t like to guess when it comes to tire pressure. If I did have the data I would suggest you run inflation sufficient to support 115% to 125% of the measured load on the rear axle. BUT you should not exceed the max inflation rating for your wheels. Depending on your specific vehicle loads and tires you may find that running the tire sidewall number might only provide 105% of the required load capacity or the “max” might put you at 130% of the measured load. You have access to the data so you can answer your own question.