My TT came with cheap tires from China. I did a few short trips and one long one, NJ to FL. I had no issues but decided to change to Goodyear Endurance. The old tires had 65 psi on the sidewall, but the Goodyears say 80 psi. I did use the TT with the tire psi at 80 but I have seen it suggested that even though it says 80 psi, I should have deflated them to 65 psi. Which is correct?
My answer to Denise is: It depends…
Ya, this sounds like a cop-out, but a key bit of information is missing.
You can skip to the bottom and just read “The Bottom Line.” But for those that want to understand how I got there, here are the details.
How much load is on tires
The important information is the answer to the question: How much load are you actually placing on your tires? The proper way to learn that is to get the RV on a truck scale with the RV fully loaded. From your comment, I am guessing you might be in NJ or spending the winter in FL, so it might not be easy to get the RV fully loaded (all your clothes, books, tools, food, water, and propane, as it might be when you are starting a trip). Our goal is to learn the maximum load you might ever place on the tires.
Also, it is best if you get the weight for each axle separately by parking with one axle on one scale platform and the other axle on a different pad. If the scale you use only has a single pad, you would need to get a weight reading with only one axle on the scale and then move the trailer so the other axle is on the pad.
Once you learn the load on the heavier axle, you should assume the load on each tire on that axle is not exactly half the axle load. There is a lot of data from thousands of RVs that suggests one end is always heavier than the other. But without individual tire load readings, we cannot know which end is heavier. So our option is to multiply the heavy axle weight by 0.52 for your RV.
With that number as your estimated max tire load, you would go to the Goodyear Load & Inflation tables. Select your size and load range tire. Then find the box with a load number that is equal to or greater than the 52% number you just calculated. The inflation shown for that box is the MINIMUM cold inflation pressure you should ever have in any of your four tires when you start to travel. I recommend that people running TPMS set the low-pressure warning level to that level of inflation.
Add 10% to minimum inflation
I also recommend you add 10% to that minimum inflation to learn what pressure you should be setting your pressure to. The 10% is a small margin that allows for day-to-day temperature changes that would affect your inflation number. Tire pressure changes by about 2% for every 10 degrees change in temperature, and I want to save you the effort of having to adjust tire inflation every day.
You may ask yourself why all this work and calculation. What we are trying to do is to ensure that no tire is ever run in overload or underinflated. The 65 psi inflation on your RV certification sticker is based on the GAWR (gross axle rate rating, or max axle load) your RV is designed to carry. In theory, tires inflated to 65 psi should be able to support the load.
But actual measurements of thousands of RVs have found that a majority of RVs have a tire or axle in overload and this is one reason so many RVs have tire failures. Another reason for the high failure rate gets technical, but it has to do with the fact that trailer tires are being dragged around every turn and corner. This side-loading results in extreme forces on the tire that are trying to tear the tire apart. I have covered the science of this force in my RV Tire Safety blog.
Sample printout from CAT scale
Here is the printout from the CAT scale showing the total load on four tires.
I start by assuming one axle has 52%, so 0.52 x 6060# = 3,152# (always round up)
and 0.52 x 3,152# = 1,639#.
We need to know the tire size, which is ST 225/75 R15, for the next step.
Tire size: ST 225/75 R15, so the GY chart says 35 psi can support 1,600#.
As I covered in my post on “Reserve Load,” you can see that normal motor vehicles run about 30% reserve or more. Since you have such a lightweight RV, I suggest you shoot for at least that level, so that gets us to 130% of 35 psi or 46 psi minimum cold inflation. Your certification sticker says 65 psi, so I would run 65 psi. Set your TPMS Low-Pressure warning level to no lower than 46 psi. You will need to review your TPMS info, as some systems can be set to a specific level while others automatically “warn” after a certain percent pressure is lost.
Inflate the GY Endurance to 65 psi cold. Ensure the low-pressure warning level of your TPMS is no lower than 46 psi.
Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.