Ms. Darlene Skelton
National Institute of Emergency Vehicle Safety, Inc.
17155 Robey Drive, Suite 101
Castro Valley, CA 94546-3852

Dear Ms. Skelton:

This responds to your letter to Mr. Barry Felrice, Associate Administrator for Safety Performance Standards of this agency, in which you asked the applicability of our Federal motor vehicle safety standards (FMVSS) to three specific scenarios. I apologize for the delay in responding.

By way of background information, under the provisions of Chapter 301 of Title 49 U. S. Code, the National Highway Traffic Safety Administration (NHTSA) has authority to issue FMVSSs applicable to new motor vehicles and new items of motor vehicle equipment. All new motor vehicles and new items of motor vehicle equipment must comply with all applicable FMVSSs in effect on the date of manufacture, and manufacturers must certify such compliance by affixing to each vehicle a manufacturer's certification label. Among other requirements, the certification label must contain the gross vehicle weight rating (GVWR) of each vehicle and the gross axle weight rating (GAWR) of each axle on the vehicle, identified in order from front to rear.

a. Your first question referred to a situation in which the GVWR exceeded the tire load ratings. Specifically, you alluded to the case of a fire truck with four rear mounted tires, each rated at 7,000 pounds (lb), that were installed on a 31,000 lb. axle. You stated that the final stage manufacturer received a letter from the tire manufacturer raising the tire inflation pressures from 100 to 110 or 115 pounds per square inch and limiting the driving to not more than 7 miles at a speed not to exceed 55 miles per hour. You asked whether such practices violated the FMVSSs.

Paragraph S5.1.2 of FMVSS No. 120, Tire selection and rims for motor vehicles other than passenger cars, provides in pertinent part:

[T]he sum of the maximum load ratings of the tires fitted to an axle shall be not less than the gross axle weight rating (GAWR) of the axle system as specified on the vehicle's certification label required by 49 CFR part 567.

In other words, the load ratings of the tires on motor vehicles other than passenger cars must be at least equal to the weight ratings of the axles on which the tires are installed. The standard makes no provision for changing the tire inflation pressures or driving at restricted speeds or limiting the distances the vehicle may travel to compensate for discrepancies in the load and weight ratings.

The facts you provided us, however, are not sufficient on which to base an opinion as to whether there has been a noncompliance in that instance. We would have to know all the facts and circumstances relevant to the tire manufacturer's alleged actions, including input from the manufacturer itself, before we could arrive at a conclusion in that regard.

b. Your second question referred to vehicles in which axles had been rerated. You cited a situation in which a manufacturer increased the GAWR of fire trucks because fire trucks do not cycle as much as tractor trailer trucks. Thus, the manufacturer increased the GAWR of fire trucks from 22,000 to 24 000 lbs.

NHTSA defines the GAWR as:

[T]he value specified by the vehicle manufacturer as the load-carrying capacity of a single axle system, as measured at the tire-ground interface (emphasis added).

A manufacturer's assigning different GAWRs to axles on different vehicles is not prohibited by our FMVSSs. In fact, manufacturers routinely assign different GAWRs and GVWRs to different vehicles based on the various equipment options and add-ons, particularly with respect to emergency vehicles. In any case, NHTSA expects that the GAWR(s) stated on the vehicle's certification label correctly reflects the manufacturer's certification that the vehicle complies with all FMVSSs applicable to that vehicle.

c. Your final question asked whether it was a violation of the FMVSS for manufacturers to take the air supply for their vehicle horns off the air supply for the vehicle's brakes. The answer is, in general, no.

FMVSS No. 121, Air brake systems, specifies performance and equipment requirements for braking systems on motor vehicles other than passenger cars that are equipped with air brake systems. The standard does not prohibit the use of air pressure from the brake air supply for the horn, but doing so could affect the vehicle's compliance with the standard.

If the horn operating off the brake air supply is installed as original equipment on a new vehicle, the manufacturer is required to certify that the vehicle complies with all applicable FMVSSs, including FMVSS No. 121. If the horn is added to a previously certified new vehicle, the person so modifying the vehicle would be an alterer who would be required to certify that, as altered, the vehicle continues to comply with all of the FMVSSs affected by the alteration. If the horn were installed on a used vehicle by a vehicle repair business, that business would not be required to attach a label or recertify the vehicle. It would, however, have to make sure that it did not knowingly make inoperative any part of a device or element of design installed on or in the vehicle in compliance with an applicable FMVSS.

In addition, hoses connected to air horns could be subject to Safety Standard No. 106, Brake hoses. They are subject to the standard if they transmit or contain the brake air pressure used to apply force to a vehicle's brakes, i.e., if a failure of the hose would result in a loss of air pressure in the brake system. If this would be the case, the hoses are "brake hoses" and must comply with Standard No. 106. If a check valve or other device is used to prevent loss of pressure, then the hose would not contain or transmit the air pressure and would not be required to comply with the brake hose standard.

I hope this information is helpful to you. Should you have any further questions or need any additional information, please feel free to contact Walter Myers of my staff at this address or at (202) 366-2992.


Philip R. Recht Chief Counsel

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