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RV Tire Safety: Are ST-type tires better because they have a higher “speed rating”?

By Roger Marble
I recently saw an RV forum post about better durability of trailer tires. The poster said: “Old trailer tires were rated to 65 mph. Newer tires are rated to 81 or 88 mph. That is a big difference. Since I upgraded to Load Range D and Speed Rating M (81 mph), my trailer blowouts have disappeared.”

My response:
Glad you are getting better durability from your trailer tires. I would not attribute all of the better durability to the higher speed rating. We are seeing that many/most ST tires manufactured since 2018 are more durable than the tires of 2000. The increase in Load Range probably gave your tire durability a good boost.

One thing you can look at is the material list molded on the tire sidewall. I think you will see that older tires did not list a layer of nylon or other material on top of the two steel belts in the tire tread. However, many of today’s ST tires list nylon.

We need to remember that “Speed Rating” is a bit misleading. ST-type tires have their load capacity formula based on a max operating speed of 65 mph since the 1970s. Otherwise their load capacity would be similar to LT-type tires.

How “Speed Rating” is established for tires

The test used to establish a “Speed Rating” is a 30-minute step-speed test designed for passenger-type tires. To pass a “speed level” a tire only has to be able to run 30 minutes without failing, after which the tire is scrap. So, clearly a tire with an 81 mph “rating” should not be considered acceptable for running many cumulative hours at 80 mph.

We really need to only use the “rating” as a measure of RELATIVE heat resistance. Also, it can be misleading to try and compare the rating on tires from company “A” with tires from company “B”. Each company will use test results from a small number of tires that are actually tested to establish the “rating” symbol for that group of tires. This is done statistically. The statistical prediction used by company “A” is not going to be the same as used by company “B”.

Also, there is no DOT test for this rating. So I doubt that you can depend on all tires of a specific speed rating to perform the same. With no federal regulation for speed performance, I do not see a reason to expect any ST-type tire to be capable of running at the stated speed for more than 30 minutes when brand-new on a perfectly smooth surface as used in the test laboratory.

Bottom Line

When shopping for new tires for your trailer, I suggest you “read the fine print.” Look at the material list for the tread area molded on the tire sidewall. An ST tire with more than just steel and polyester listed, e.g., Polyester + 2 layer Steel + 1 layer Nylon, will probably be more durable than one without the nylon when operated at speeds above 65 mph. IMO a “speed rating” of L (75 mph) should be more than sufficient for RV trailer use.

 

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT998

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Richard
4 months ago

How long does it take for the tires to reach “cold” stage after you drive it for the day? I check my tire pressure every morning but sometimes I would like to leave the campground before quiet hours are over. If a tire requires air, I will not run my compressor during quiet hours even though it is of the type that makes very little sound. Is it possible to check them 5-6 hours after you quit driving for the day and be able to get an accurate cold tem reading.

Roger Marble
4 months ago
Reply to  Richard

We generally figure 2 hours is long enough. Just make sure the Sunshine is off the tire too.

Richard
4 months ago
Reply to  Roger Marble

Thank you. Blessings and safe travel

Lee Ensminger
4 months ago
Reply to  Roger Marble

Roger, thanks for mentioning that tires need to be out of the sun for a correct measurement. Some people don’t realize that sunshine on a black tire raises the temperature and thus, pressure.

John
4 months ago

I have a 2006 Winnebago Aspect 26A that we bought in April 2012. I had 2 blowouts and one tire where the ply was separating, but I caught it before a blowout. All in the same position, passenger, rear, inner dualie. I think the culprit was the exhaust went out the side of the RV, just forward of the tire well (actually in the forward part of the well). Recently had a shop re-route the exhaust, it now continues aft then comes out just beside the generator’s exhaust pipe. Hopefully this will have solved this. I think mine are Load Rating R, Speed Index 112. Don’t know what all that means.

chris
4 months ago
Reply to  John

I think everyone should have a TPMS to at least have a chance of knowing *before* a tire blows.

John
4 months ago
Reply to  chris

TPMS are not perfect. I’ve had them read the correct air pressure and the tred was gone, nothing to hold air but it still said it had air in it.

Roger Marble
4 months ago
Reply to  John

John. Like anything as complex as a TPMS there can be problems. Here is a post from my blog on how to test your TPMS. You should do this at least once a year. I have also found that not all TPMS have been properly programmed. I have found that TPMS are accurate to about +/- 2psi. Remember they are designed to MONITOR a pressure change. Your digital hand gauge should be your “Master” when setting the cold pressure.

WEB
4 months ago
Reply to  Roger Marble

Roger, your “programmed” link above is bad.  😳 

Roger Marble
4 months ago
Reply to  WEB
Gary W Mayberry
4 months ago
Reply to  John

Same thing happened to my son last summer. Tread was gone and ripped up the side/fender of his 5’er but still held the air pressure needed.

Roger Marble
4 months ago

One way to possibly learn if your tires are in the process of developing a belt separation (yes TPMS do not monitor belt separations) is to do a “Free Spin” inspection once a year or each 2,000 miles whichever comes first. Info on my blog since 2014.

https://www.rvtiresafety.net/2014/08/how-do-i-inspect-my-tires.html

Roger Marble
4 months ago
Reply to  John

John, I suggest you re-read the tire sidewall. No such thing as “Load Rating R”

I gonna guess your Winne is a Class-B type RV The tires probably are something like LT225/75R16 Load Range E With a Speed Rating of R The 112 is the tire Load Index. Different manufacturers may show slightly different Load Index numbers but usually, there are two numbers Like 112/115. It may be best to snap a picture of the actual tire size on the sidewall. You also have a Certification Label on the driver door jam that should have the specific info on your tires. If you can email me the picture of the tire sidewall I will be happy to translate all the numbers and letters. Tireman9 at Gmail.com

Thomas D
4 months ago

3blowouts in one day. They were on a trailer
LT style. American made. 4 years old . Never driven more than 70 mph. 750#or more per tire reserve. Go figure!

Roger Marble
4 months ago
Reply to  Thomas D

Well, I would like to see some pictures (email to me) and I might be able to offer an opinion. I would suspect a faulty tire valve core, or no metal valve cap. You didn’t mention having a TPMS. What are the scale weights for each axle?

Tommy Molnar
4 months ago

I’ve decided that 63 mph is my ‘magic number’ and set my cruise for that. That makes it easy for those wanting to go 80 mph to get around.

chris
4 months ago
Reply to  Tommy Molnar

I find it’s the weekend warrior toy haulers that do that. I try to avoid Sunday afternoons.

Glenn
4 months ago
Reply to  Tommy Molnar

That’s funny. Last year back and forth cross country that is the exact speed I set my cruise at. Seemed like the “sweet spot” also.

Roger Marble
4 months ago
Reply to  Glenn

That’s my speed too.

chris
4 months ago

Blowouts? I’ve had ONE in 18 years. Is this a constant problem for some? How fast do some people want to tow?

Last edited 4 months ago by chris
Wayne C
4 months ago
Reply to  chris

I tow fast. Sometimes 75 across Nevada in the winter to get where it’s warm. I went from 15” to 16” wheels and from LR D to LR E LT tires which provides about 35% load margin. I slow down when the pavement is hot but I travel in the non summer months. I tow an 8 thousand pound 5th wheel trailer with an 8 thousand pound diesel pickup. I haven’t had any tire problems and the rig handles so well it’s a pleasure to drive.

Drew
4 months ago
Reply to  Wayne C

Wayne, I think you should consider traveling at a more conservative speed. Things can happen very quickly the faster you go (especially when towing).

David Stone
4 months ago
Reply to  Wayne C

Wayne,
I am with you. I tow 75 or 80 mph.
I towed my 10k pound Formula 382 on 265E16s from Phoenix to Indiana. One failure in Illinois (almost home ) Seems that the Super Duty feels good around 80. Couple of years later I had another failure after only 2 hours highway time, (Shorty after fuel and back on the tollway).
I couldn’t say if tire age or heat was the culprit as the tread seperated yet was still inflated.
I have two spares, bottle jack and an impact. Besides the alligator spanking my fender it wasn’t too big of a problem on the tri axle trlr.
I tow my snowmobiles at 75 or 80 also (usually keep up with the flow of traffic )

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