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CWtheMan · 08-03-2018, 12:12 PM

There seems to be a bit of misunderstanding here. The Federal Motor Vehicle Safety Standards (FMVSS) are a mandatory safety net, written by the National Highway Traffic Safety Administration (NHTSA) to insure all vehicles - including all RV trailers - are built in compliance with minimum safety standards.

The final step a RV trailer manufacturer must do before it leaves the factory is certify that all those standards have been met.

Minimums and maximums are confusing to many owners. Especially with tire fitments.

First I’m going to establish a couple of facts.

The axles on your trailer may have two GAWR values. The axle manufacturer builds and certifies their axles to a weight value. It is defined as its Gross Axle Weight Rating (GAWR). The axle manufacturer is required to display that weight rating on each individual axle and the do so with a tag attached to each axle. Here’s the catch. Vehicle manufacturers have the authority from NHTSA to set GAWR values below the axle manufacturer’s values. Therefore, the official GAWR values for any RV trailer can only be found on its federal certification label. A common setting found on many older Keystone RV trailers was for axles manufactured with a 5200# rating from their manufacturer to be de-rated by Keystone to 5080#. They did that so they could mount tires rated at 2540# on those axles. That’s just one example. Numerous 7000# axles were also de-rated to accommodate Marathon tires that were rated at 3420#. That’s just some of the minimums Keystone complied with.

The standard for determining a RV trailer’s GAWRs has always been determined by the maximum weight it has been designed to carry, as determined by the vehicle manufacturer, in accordance with minimum standards set by FMVSS. Remember, the build target is the GVWR. To establish a trailer’s GAWRs the vehicle manufacturer does this simple math, which is a mandatory step in vehicle certification. They must set a recommended tongue/hitch weight. When that hitch weight is added to the total vehicle certified GAWR values they must not be less than GVWR. It’s the only time that tongue weight will be used unless an owner can perform the magic needed to duplicate it with a balanced load in their trailer. The weight difference between GVW and GVWR is cargo. Again, you must remember, the trailer GVW is not sitting on the tires. A percentage is always setting on the tongue/hitch of the tow vehicle or its landing gear.

Original Equipment (OE) tire selection for your RV trailer is driven by FMVSS and is the sole responsibility of the vehicle manufacturer. First and foremost is the absolute minimum standard, to provide a load capacity that will support the maximum load capacity of the vehicle certified GAWR axles. The bottom line is a provision in the FMVSS that reads - in part like this; The size designation and the recommended cold inflation pressure for those tires such that the sum of the load ratings of the tires on each axle is appropriate. The word appropriate at the end of that statement establishes the fact that the OE tires are the minimum requirement for that fitment and all subsequent replacements must have a load capacity equal to that fitment.

Recently, The Recreational Vehicle Industry Association (RVIA) has made a recommendation for all RV trailer tires to have, at the least, a 10% load capacity reserve above the trailer’s GAWR. IMO most RV trailer manufacturer’s will do just that and add the increased cost to them to the consumers.

Something to consider when researching tire safety is your trailer’s owner manual. Everything about tire safety found in those manuals are there in accordance with NHTSA mandates.

When linking standards and regulations to form a conclusion, don’t forget to add CFR 49 part 567 (Certification) to your list of references.

Here is a sample of a research topic. They all say it a little differently but the bottom line is the same. It’s a tire industry standard driven by a FMVSS minimum standard.

Goodyear: Never fit tires to a vehicle that have less load carrying capacity than required by the Original Equipment Manufacturer.

Michelin: Never choose a tire that is smaller in size or has less load-carrying capacity than the tire that came with the vehicle.

Cooper: The new tires must have a load carrying capacity equal to or greater than the maximum load carrying capacity specified on the tire placard on the vehicle.

Toyo: Any replacement tire must be of a size and load range that will offer equal or higher load carrying capacity compared to the original equipment (OE) tire on the vehicle.

This is another reference from the USTMA that is useful when discussing tire inflation pressures.

“Inflation pressure recommendations may be determined
based on the tire manufacturer’s specifications, which define
the amount of inflation pressure necessary to carry a given
load. These inflation pressures may differ from those found
on the vehicle tire placard or certification label.”

“However, never use inflation pressure lower than specified by
the vehicle tire placard, certification label or owner’s manual.
Nor should inflation pressure exceed the maximum pressure
molded on the tire sidewall.”

08-03-2018, 12:12 PM	#80
CWtheMan Senior Member Join Date: Feb 2010 Location: Taylors, SC Posts: 3,031	Gawr There seems to be a bit of misunderstanding here. The Federal Motor Vehicle Safety Standards (FMVSS) are a mandatory safety net, written by the National Highway Traffic Safety Administration (NHTSA) to insure all vehicles - including all RV trailers - are built in compliance with minimum safety standards. The final step a RV trailer manufacturer must do before it leaves the factory is certify that all those standards have been met. Minimums and maximums are confusing to many owners. Especially with tire fitments. First I’m going to establish a couple of facts. The axles on your trailer may have two GAWR values. The axle manufacturer builds and certifies their axles to a weight value. It is defined as its Gross Axle Weight Rating (GAWR). The axle manufacturer is required to display that weight rating on each individual axle and the do so with a tag attached to each axle. Here’s the catch. Vehicle manufacturers have the authority from NHTSA to set GAWR values below the axle manufacturer’s values. Therefore, the official GAWR values for any RV trailer can only be found on its federal certification label. A common setting found on many older Keystone RV trailers was for axles manufactured with a 5200# rating from their manufacturer to be de-rated by Keystone to 5080#. They did that so they could mount tires rated at 2540# on those axles. That’s just one example. Numerous 7000# axles were also de-rated to accommodate Marathon tires that were rated at 3420#. That’s just some of the minimums Keystone complied with. The standard for determining a RV trailer’s GAWRs has always been determined by the maximum weight it has been designed to carry, as determined by the vehicle manufacturer, in accordance with minimum standards set by FMVSS. Remember, the build target is the GVWR. To establish a trailer’s GAWRs the vehicle manufacturer does this simple math, which is a mandatory step in vehicle certification. They must set a recommended tongue/hitch weight. When that hitch weight is added to the total vehicle certified GAWR values they must not be less than GVWR. It’s the only time that tongue weight will be used unless an owner can perform the magic needed to duplicate it with a balanced load in their trailer. The weight difference between GVW and GVWR is cargo. Again, you must remember, the trailer GVW is not sitting on the tires. A percentage is always setting on the tongue/hitch of the tow vehicle or its landing gear. Original Equipment (OE) tire selection for your RV trailer is driven by FMVSS and is the sole responsibility of the vehicle manufacturer. First and foremost is the absolute minimum standard, to provide a load capacity that will support the maximum load capacity of the vehicle certified GAWR axles. The bottom line is a provision in the FMVSS that reads - in part like this; The size designation and the recommended cold inflation pressure for those tires such that the sum of the load ratings of the tires on each axle is appropriate. The word appropriate at the end of that statement establishes the fact that the OE tires are the minimum requirement for that fitment and all subsequent replacements must have a load capacity equal to that fitment. Recently, The Recreational Vehicle Industry Association (RVIA) has made a recommendation for all RV trailer tires to have, at the least, a 10% load capacity reserve above the trailer’s GAWR. IMO most RV trailer manufacturer’s will do just that and add the increased cost to them to the consumers. Something to consider when researching tire safety is your trailer’s owner manual. Everything about tire safety found in those manuals are there in accordance with NHTSA mandates. When linking standards and regulations to form a conclusion, don’t forget to add CFR 49 part 567 (Certification) to your list of references. Here is a sample of a research topic. They all say it a little differently but the bottom line is the same. It’s a tire industry standard driven by a FMVSS minimum standard. Goodyear: Never fit tires to a vehicle that have less load carrying capacity than required by the Original Equipment Manufacturer. Michelin: Never choose a tire that is smaller in size or has less load-carrying capacity than the tire that came with the vehicle. Cooper: The new tires must have a load carrying capacity equal to or greater than the maximum load carrying capacity specified on the tire placard on the vehicle. Toyo: Any replacement tire must be of a size and load range that will offer equal or higher load carrying capacity compared to the original equipment (OE) tire on the vehicle. This is another reference from the USTMA that is useful when discussing tire inflation pressures. “Inflation pressure recommendations may be determined based on the tire manufacturer’s specifications, which define the amount of inflation pressure necessary to carry a given load. These inflation pressures may differ from those found on the vehicle tire placard or certification label.” “However, never use inflation pressure lower than specified by the vehicle tire placard, certification label or owner’s manual. Nor should inflation pressure exceed the maximum pressure molded on the tire sidewall.”