I’m sure that very few people enjoy thinking about tires, but there are some basics that need to be considered when planning to purchase new tires. This post is just a short summary.
Here are some facts on proper tire applications.
First: RV manufacturers are required, by federal law, to apply tire information placards aka certification labels that state both the tire size/type and the minimum inflation you need to run to support the stated load. This same type of information can be found on a Smart car, an F350 dually, or a 40-foot Class-A RV.
Placards for RVs
Placards for RVs will look something like this one and have information on vehicle loads, tires and inflation:
Trailers have this label on the outside on the driver’s side toward the front. If you can’t find it, your dealer should be able to point it out. If it is missing or faded, the RV manufacturer should be able to provide a replacement. For the last few years, Class A’s have this label inside near the driver’s left elbow.
Passenger car (P) and light truck (LT) labels will look like this and are located on the driver’s side door jamb. Passenger cars do not include load information as you are not expected to load it with more than the 150 lbs. per passenger and maybe 50 lbs. luggage.
Types of tires
Second: There are a number of “type” tires that the RV owner might run into. You should not be changing the “type” tire without a lot of research and are in contact with both tire companies and RV manufacturer.
P-metric: These are primarily intended for passenger car applications. According to industry standards, they can be used in other applications but their load capacity should be adjusted down by 10% if on anything other than a standard car. This -10% adjustment in load capacity applies to SUV’s, station wagons, pickups, vans and any RV.
P-metric tires are capable of sustained 85 mph or greater. For example, a P235/75R15 105S is capable of 2,028 lbs. @ 35 psi in car application. Special note: The load is reduced to 1,843 lbs. at 35 psi in light truck, station wagon, van, trailer or bus operation. In “105S”, the “105” is the Load Index and the “S” is the speed rating, in this case 112 mph. I do not find any information on dual application of P-metric tires in industry standards, which I interpret as “Not Approved’.
LT-metric: These are primarily intended for light truck application. Most have a radial construction, as in LT235/75R15 104/101R LR-C. This tire is rated at a Max of 106 mph and at 50 psi can carry 1,985 lbs. single or 1,820 lbs. when mounted in dual application. For reference, at 35 psi it can carry 1,530 lbs. single or 1,390 lbs. in dual. Special note on speed: If an LT tire is operated above 65 mph, its cold inflation must be increased by 10 psi but never above the tire Max cold. If you cannot increase the inflation, you must reduce the load to meet your application needs.
ST-type: These are special trailer tires and come in a more limited range of sizes. Many ST tires are not radial (R) but carry a “D” in their size, indicating diagonal or bias construction instead of the “R”. An ST225/75R15 is rated at a Max of 65 mph and 2,150 lbs. in single application at 50 psi minimum. The 65 mph limit is a concession to compensate for the load increase versus P-type tires.
I do not find any information on dual application of ST tires in RV Industry standards. ST-type tires are not approved on vehicles that carry passengers. They can also have a “speed symbol” that is considered as a relative measure of resisting heat damage for running higher speeds. The test for the “speed symbol” only requires a tire be able to run 10 minutes at the stated speed.
High flotation tires, such as 31×10.5R15LT 109R LRC, are sometimes considered for the pickup tow vehicle. This tire is rated at a Max of 106 mph and at 50 psi can carry 2,270 lbs. It is 30.52″ OD (outer diameter), while the LT235/75R15 is listed at 28.9″ OD. I do not find any information on dual application of high flotation tires in Industry standards.
Class-A RV TBR (Truck Bus Radial) tires, such as 295/80R22/5, are rated at 7,830/6,940 lbs., single/dual at 125 psi. Goodyear, Bridgestone, Michelin and other tire companies have a stated 75 mph Max speed for TBR tires in RV application. For this application, it is imperative that you check the tire manufacturer’s data sheets for the details on your specific application and size.
Most important
Third and most important: The “MAX” on the tire sidewall is the COLD PSI that delivers the highest or “MAX” load capacity for that tire. IT IS NOT THE MAXIMUM PSI THE TIRE CAN TOLERATE.
Tires made to European standards do not have the leading letter, i.e., P, LT or ST. They have standard passenger car tires and “commercial”-type tires. This may result in some confusion. So, if you are considering switching the tire “type” and from U.S. to European standards, there is additional research you MUST do.
This post is just a summary. You can find more details in my weekly posts on www.RVtravel.com under the “Maintenance/Repair” header. And you can review any of the 250+ posts in my RV Tire blog at www.TireSafety.NET.
Roger Marble
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Thank you ,Roger. Very informative.
An excellent summary IMO, Roger.
Thank you for the review, Roger! Have a great week and safe travels!