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Monday, December 19, 2022

Can I change from 65 psi trailer tires to 80 psi GY Endurance tires?

 Denise asked:

My TT came with the cheap tires out of China. I did a few short trips and one long, NJ to FL. I had no issues but decided to change to the Goodyear Endurance. The old tires had a psi of 65 but the Goodyears have a psi of 80. I did use the TT with the tire psi at 80 but I have seen that even though it says 80psi I should have deflated them to 65psi. Which is correct?

The answer 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.
That being 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 as there is a lot of data from thousands of RV that suggests one end is always heavier than the other but without individual tire load readings, we can not know which end is heavier. So our option is to multiply the heavy axle weight by 0.51. With that number as our 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 51% number you just calculated. The inflation shown for that box is the MINIMUM cold inflation pressure you should ever have in any of your 4 tires when you start to travel. I recommend that people running TPMS set the low-pressure warning level to that level of inflation.
I also recommend you add 10% to that minimum inflation to learn what pressure you should be setting your pressure too. The 10% is a small margin that allows for day-to-day temperature changes that would affect your inflation number as tire pressure changes by about 2% for every 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 (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 is trying to tear the tire apart. I have covered the Science of this force in my RV Tire Safety blog.

Having given you more information than you wanted, and knowing that you may not be able to get your RV on a scale in the near future, I will recommend that until you can get the loads measured that you run your Goodyear Endurance tires at 70 psi. Once you do the calculations you may be able to run lower than 70 or you may need to run higher.

  That's a good start. the 6060# would be both axles so we need to be conservative.
I start by assuming one axle has 51%  .51x 6060 =  3,091 (always round up)
and .51x 3091 = 1,577# 
I need to know the tire size for the next step
Tire size: ST 225/75 R15 so the GY chart says 30 psi can support 1,600#
As I covered in my post on "Reserve Load" you can see that normal motor vehicles run about 30%. Since you have such a light weight RV I suggest you shoot for at least that level, so that gets us to 50 psi minimum cold inflation.  Your Certification sticker says 65 psi so I would run 65 psi and set your TPMS Low Pressure warning level to no lower than 50 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 lost.

Bottom Line
Inflate the GY Endurance to 65 psi cold. ensure the low pressure warning level of your TPMS is no lower than 50 psi.

Saturday, December 10, 2022

Six blowouts! Help! "I'm at my wit's end"


Here is a series of posts on tire blowouts from an RV forum. I am inclined to think that the experiences are not that unusual. This is a long post, but I thought that giving the background, and my recommendations, might help others that have tire problems know that they are not alone.

John wrote:

"I'm at my wit's end on tires. 2000 Winnebago Journey, I have blown out 6 tires since I have had this unit, over 10 years, and done over $30,000 of damage to it. Just got back from a 2,000-mile trip to Colorado and back, after blowing two tires in two days. I have tire sensors, the unit is always kept garaged, no sun on tires, always check pressures, have moved up to 255/70R22.5, which is slightly larger than stock but a lot easier to find. I run 110 psi cold, which is less than the listed 120 PSI. Maybe the age of the tires is getting me, but none are older than 6 years. I am going to start buying two tires every 2 years, to keep the age down. Any other suggestions?"

John continues in another post:

"This is really getting annoying to find tires on the road and then repair all the damage. Thank goodness for insurance. The original tires were 235/80. I upgraded to 255/70, and still have over an inch between tire sidewalls. The tires are not rubbing. It would appear that I have overlooked a very important detail in my quest, and that would be the tire pressure chart many have referenced. I have been running pressure at 110 or slightly below, thinking that might give me a softer ride, when in fact, at that pressure I do not have much "free space" between the load capacity of tires, and the weight of the motorhome.

My coach has a GAWR on the rear of 15,500, and with my tires at 110 psi, the load capacity is uncomfortably close to capacity. I never really understood pressure charts, but do now and will abide by them. In addition, I am going to put my coach in a weight reduction program by removing things I do not need to carry all the time. That, and a program to ensure my tires are not over 6 to 8 years old, and spinning my tires instead of sitting on my concrete slab for months at a time should cure my ills. Thanks to each for your suggestions and help."

There were some replies and suggestions from myself and other RV owners concerning the RV weight.

John posted in reply:

Okay, I finally got my rig weighed, and here are the results. My GVWR on the rig is 24,850 lbs. The weight of my rig with just me was 7,760 lbs. front axle and 13,460 lbs. rear axle for a total of 21,220 lbs.

I figure loaded with people and stuff add 3,000 lbs. with 75 % on rear axle, so that would be 15,710 rear and 8,510 front axle, for a total of 24,220 lbs. weight when loaded.

This is pretty close to GVWR, but the front axle rating is 9,350 and the rear axle rating is 15,500, for a total of 24,850.

Tires are rated 4,670 lbs. @ 110 psi X 4 = 18,688 lbs. rear and front 5,205 lbs. @ 110 psi X 2 = 10,410 lbs. front for a total capacity of 29,098 lbs. for the coach.

So I am close to GVWR but slightly under, closer to limits on axle capacity, but axles aren't breaking, tires are. Seems that I am within limits, and would be even safer if I air tires up to 120 psi, which would give me 5,070 X 4 =20,280 on the rear and 11,020 on the front. Tires will carry far more than the coach is rated for, so my problem must be just old tires. Thanks for all the suggestions on this.

Another RV owner, Mike, offered:

John, do yourself a favor and don't guess what your loaded weight is. On your next trip, at your 1st gas up, go to a station with a CAT Scale, and immediately get your MH weighed at it's full travel weight. You're at 6 blown tires, and counting, with oversized tires and super high pressures. You need more than a back-of-the-envelope estimate of your real travel weight. Somewhere in that combination is a problem unique to your setup.

At this point, I added another and hopefully a final post:

John, it's good that you got some info on the weight, but I think Mike's suggestion to get actual weights and not guess, especially since you are close to the limit, is spot on.
Another point is that GVWR is not what you need to be concerned with. Ideally, you should try and learn the load on each tire position aka "4-Corner Weights". The reason for this is that tires on one end of an axle do not share the load on the other axle end. The load is almost never split 50/50. One end of an axle could have 200# to 500# more than the other end of that axle. I have seen an extreme case with one end supporting 1,000# more than the other end.

To learn the load on individual tire positions you can check with building supply or gravel sellers. But until you can get "4-corners" I suggest you confirm your GAWR from your Certification Sticker and go to a truck scale. But be sure to get individual axle weights as there will be a significant difference Front vs. Rear. I have a few posts on "4-Corner Weights" on my blog you can read HERE to help you understand how and why this is the best system of weighing.


1. For each axle weight, I suggest you assume there is a side-to-side load split of at least 51/49% and use the 51% number for inflation selection until you get actual 4-Corner weights.

2. Look on the Load Inflation Tables to learn the MINIMUM inflation for all the tires on that axle based on the 51% number. Yes, all tires on an axle should be inflated to the same level.

3. For your "cold" inflation, I recommend you run at least 10%, with +15% more than the inflation shown in the tables better, if possible. This will provide a larger "Reserve Load". BUT DO NOT EXCEED the max inflation number for the wheels which might be 120 psi for your coach. So do a bit of research.

4. I also suggest you set your TPMS low pressure to the psi in #2 above.

Hopefully, this confirmation of load and a little bit of math and adjusting your inflation will solve your tire problems.

Friday, December 2, 2022

Short post on Who makes that tire?


I am not aware of a similar list for trailer tires. The best you can do is use Google and post the Question " Who makes TowMax tires" for example. You will get the reply that "Towmax brand is owned by TBC brands. If you have a tire read the DOT serial. The first 3 characters identify the location and name of the tire plant where the tires were made. Here is the link  

Complete List of Tire DOT Plant Codes - With All Old and New ...