During a road trip as a passenger in a large motor coach recently, I realized how heavy the load was and decided to check out the tires. As I suspected, they were the same size (295/75R22.5) as the most popular fitment on Class 7 and 8 commercial trucks. Closer inspection, however, revealed that this new RV was fitted with the Load Range H version of this tire size, while the vast majority of trucks ride on Load Range Gs.
The major differences are increased load capacity of about 430 lb. per tire and increased inflation pressure to support that load. Similar differences can be found on some applications of sizes 11R22.5 Load Range H (vs. the more popular Load Range G), and the 245/70R19.5 Load Range G vs. Load Range F.
A more complex tire situation has developed in the Class 4 through 6 chassis market and in certain types of towable recreation vehicles and commercial trailers. A number of the latter are typically fitted with tires of 15- or-16-in. rim diameter. The wheelhouse area on these vehicles is often kept as small as possible to maintain a low center of gravity for stability, and to allow the largest amount of interior space consistent with adequate load carrying capacity.
Most of the smaller units, which may have been fitted with passenger tires in past years, are now equipped with 15- or 16-in. special ST metric tires designed specifically for trailers in highway service. These tires have an “ST” prefix in front of the size (e.g., ST 235/80R16) and have load/inflation ratings different from their passenger-car or light-truck counterparts with the same basic metric sizing.
Many of the “ST” tires and some of the “LT” prefix light-truck tires used in recreational and commercial trailer applications require higher rated, more heavy-duty, wheels and inflation valves than their lighter duty counterparts. One such example is an LT 235/85R16 Load Range G tire, designed for commercial trailers. This tire can carry as much as 3,750 lb., but requires a cold inflation pressure of 110 psi and, therefore, must be used only with a special high-strength wheel.
Since this basic tire size is commonly fitted to larger pickups, referred to as 3/4- or 1-ton models, with single rear tires, and these are popular tow vehicles for the larger trailers, it's possible to have the same size tires on the truck and trailer. However, heavier wheels and higher inflation pressures would be mandatory for the trailer, and the truck tires would have insufficient load capacity for the trailer application.
The consequences of choosing the wrong tire are serious enough that you should consider the following:
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Check the vehicle placard or vehicle owner's manual, to determine the correct Tire Size and Load Range recommendation. If in doubt, contact the OEM to determine the proper Tire Size, Type Prefix, and Load Range. Tires with an alphabetically lower Load Range should never be substituted for a higher specified Load Range.
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Be aware that the Tire Type Prefix is an integral part of the tire size description. Tires with different prefixes have different load and inflation ratings and should not be substituted or mixed.
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Make sure the wheel/rim load and inflation ratings are compatible with load and inflation requirements.
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Check that the inflation valve is rated for the inflation pressure of the tire being used. This is especially important for 15 and 16 in. applications, where snap-in valves are used for lower inflation pressures and clamp-in valves are required for higher inflations.
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Maintain inflation pressures that are recommended by the vehicle OEM and are compatible with the ratings on the tire sidewall.
