Size Isn't All That Matters - Tire Review Magazine

Size Isn’t All That Matters

e and the increased inflation pressure required to support that load.

Similar differences can be found on some applications of sizes 11R22.5 Load Range H and the more popular Load Range G version, and with the Load Range G vs. F versions of a 245/70R19.5. Generally, the wheels used for these tire sizes are designed with enough reserve capacity for the higher loads and inflations of optional heavier duty tires, but this should always be checked if replacing either component.

A somewhat more complex tire situation has developed in the Class 4 through 6 chassis market, and especially with certain types of towable recreational vehicles and commercial trailers.

A number of commercial trailers are typically fitted with 15- or 16-inch wheel diameter tires. 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 greatest amount of interior space possible given the load carrying capacity.

Most of the smaller units, which may have been fitted with passenger tires in the past, are now equipped with 15- or 16-inch "special ST metric" tires designed specifically for highway service trailers. The tires have a "ST" prefix in front of the size (ST 235/80R16) and have load/inflation ratings different from their same-sized passenger/light truck counterparts.

The growing popularity of larger wheel diameter tires – from 16 to 20 inches ®“ on many performance car models and some pickup trucks further clouds the picture.

Many of the ST tires and some of the LT prefixed light truck tires used in RV and commercial trailer applications require wheels and valve stems that are rated higher than typical passenger/light truck applications.

One example is the LT 235/85R16 Load Range G tire, designed for commercial trailers. This tire can carry as much as 3,750 pounds, but requires a cold inflation pressure of 110 psi, so it must be used only with a special high strength wheel.

Since this same basic tire size is commonly fitted to larger GVW pickup trucks that are popular tow vehicles for the larger trailers, it is quite possible to have the same size tires on both the truck and trailer. But heavier wheels and higher inflation pressures would be mandatory for the trailer; the truck’s tires would have insufficient load capacity for the trailer application.

Back on the RV front, it should be noted that some motor home coaches, usually the larger bus-like units, may call for different tire sizes and/or load ranges for the front vs. rear axles.

Check and Check Again

With all of this complexity and the resulting possibility of misapplication, it’s a good idea to always check several important information sources. Suggestions from several industry professionals include:

®′ Check the vehicle placard – if not available, check the vehicle owner’s manual ®“ to determine the tire size AND load range recommendation of the vehicle maker. If any doubt or question exists, contact the vehicle maker to determine the proper tire size, tire type (prefix) and load range. Tires with a lower load range should never be substituted in an application that calls for a higher specified load range.

®′ Be aware that the tire type prefix (e.g. P, LT, ST) is an integral part of the tire size description. Tires with different prefixes have different load and inflation ratings and should not be substituted for, or mixed with, differently prefixed tires.

®′ Always check the wheel/rim load and inflation ratings to be sure that they are compatible with the tire load and inflation requirements for the application.

®′ Make sure that the valve is rated for the inflation pressure of the tire being used. This is especially important for 15- and 16-inch applications, where snap-in valves can be used for lower inflation pressures and clamp-in valves are required for higher inflations.

®′ Do not assume that the tires on used vehicles are the correct ones for the application. The previous owner may have replaced one or more tires previously.

®′ Do not mix and match radial and bias-ply tires.

®′ Maintain inflation pressures that are recommended by the vehicle maker and are compatible with the ratings on the tire sidewall. If there is any question, contact the OEM or a qualified tire industry professional.

Lest you think that all of this sounds incredibly confusing and impractical to administer, consider that some in the heavy vehicle maintenance industry have been dealing with similar complexity for years. Fire trucks, mobile cranes, and ready-mix concrete trucks are good examples.

But today more and more motor homes and trailers – even some emergency equipment, like ambulances ®“ are being fitted with tires that may be inadequate for the job they are being asked to do, or tires that are specifically designed for that load application.

Keep these customers safe by paying close attention to all these factors.

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