Sunday, February 5, 2023


RV Tire Safety: What “size” are your tires? Part 3 – “TBR” (on Class A RVs)

By Roger Marble
This is Part 3 of a 3-part series.
Finally, we move to Truck-Bus Radials aka “TBR” tires. These are seen on all the heavy trucks on the highway, and most Class A RVs. Generally, these are considered commercial-type tires and not a consumer-level product. If you have and need this type of tire, it is expected that you have a deeper level of knowledge about tires. Most of these tires come on 19.5 or 22.5 size wheels. They do not have a letter preceding the size description and may be something like 255/70R22.5 139/134 LR-G.

These tires seldom come with a Speed Symbol, but if you review the Data Book from the tire company that make TBR tires, you will see that they specify 75 mph as the maximum operating speed. The double Load Index numbers 139/134 relate to the single and dual application, and the Load Range letters continue to identify the normal upper level of cold inflation pressure. I suggest you look at the actual load capacity numbers in pounds, as the “index” is a range and you could end up with slight loss in load capacity.

There is a lot more information on tires available in tire Data Books and Industry Standards organizations. Since “TBR” or commercial tires are generally not considered “consumer” items, you may need to educate yourself more about the loads and inflations and speed ratings for these tires, unless you are getting the same brand, size and Load Range as came as original equipment on your RV.

Be careful about information you are relying on for what size tire

You do need to be careful about the source of the information you are relying on. I have found errors on some listings on the internet. Remember the person you are ordering tires from on the internet may not have much or even any actual hands-on experience with tires, as some sellers are not much more than order takers.

The technical data is available in tire industry publications and can normally be relied upon as accurate. But, even there, the information may not be aimed at the specific and sometimes unique needs of the RV community. There are many forums on the internet with hundreds of self-appointed “experts.” We need to be careful, as just having used or sold tires for decades does not mean that all the information from that person can be relied upon as 100% accurate.

Be sure you get the correct size tire

Recently I have seen the introduction of 17.5-size tires on a couple of large 5th wheel RVs. The tires were 235/75R17.5 LR H with a Speed Symbol of L (75 mph Max). If you are considering a move to 17.5, be sure you get the correct tires as some in that size are only rated for 62 mph Max – which would be a J Speed Symbol. There might be others at K (68 mph Max).

Please remember that if you are changing tire size or Load Range, you must do additional research to ensure you are getting the tires you want and need. If you are going from 16” LT-type tires to a TBR 17.5 tire, you also will be changing wheels. That introduces another level of complexity, as wheels have a number of critical measurements other than diameter and width – there is also offset and center bore. Even the lug nuts might need to be changed.

This is the 3rd and final part of this overview of tires used in RV application. You can find Part 1 here and Part 2 here.

Have a tire question? Sign up for Roger Marble’s new Facebook Group: RV tire news, information and discussion, hosted by and moderated by Roger. He’ll be happy to help you.

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1 year ago

One thing that was pounded into our heads as Class 7/8 truck mechanics at Ryder was to always use a remote filling hose and to never air up a tire that rode flat (below 90 psi) without doing it remotely in a cage. A zipper tire that explodes can easily kill you.

I worked with a guy at a dealership that (yes,THAT guy) who insisted on putting 16.5″ tires on 16″ rims, because they were “free” ( warranty returns). I would have nothing to do with it and warned him many times. He bumped a tire putting it on the rear axle, the explosion knocked the truck off the lift and blew him 8+ feet away.

We thought he was dead! Luckily he only broke a wrist and got to walk around for a few weeks with a cast on, while 50+ guys shook their heads at him every time they saw him.

When working on 17.5 and 22.5 tires with inflation pressures starting at 80 psi, please use a remote attachment chuck, coiled hose, and stand 20+ feet away with your gauge while filling tires.

1 year ago

Speed Limit, what speed limit. Raise your hand if you have never been passed by a big rig going at least 80 mph.

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