Finally, we move to Truck-Bus Radials AKA “TBR” tires as 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 OE on your RV. 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 on-hands 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.
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 which introduce 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.
OK this is the 3rd and final part of this overview of tires used in RV application. You can find Part 1 HERE at https://www.rvtiresafety.net/2021/09/what-size-are-your-tires-part-1-of-3.html
and Part 2 here at https://www.rvtiresafety.net/2021/10/what-size-tires-do-you-have-part-2.html.