Mr. Paul Giordano
New Jersey Motor Vehicle Commission Bus Unit
225 East State St. (4E)
P.O. Box 680
Trenton, NJ 08666-0680
Dear Mr. Giordano:
This letter responds to your request for clarification regarding the relationship between the certification label and the tire selection requirements for school buses with a gross vehicle weight rating (GVWR) of more than 10,000 pounds under Federal Motor Vehicle Safety Standard (FMVSS) No. 120, which relates to tire selection and rims. We are pleased to provide the following clarification.
We begin by clarifying NHTSAs regulatory authority. NHTSA has the authority under
49 U.S.C. Chapter 301 to prescribe Federal motor vehicle safety standards (FMVSSs). The FMVSSs are applicable to new motor vehicles and new motor vehicle equipment. NHTSA has limited authority to regulate changes made to a vehicle after its first retail sale. There is a make inoperative provision (49 USC 30122(b)) that prohibits manufacturers, distributors, dealers, or motor vehicle repair businesses from knowingly making inoperative, in whole or in part, any part of a device or element of design installed on or in a motor vehicle in compliance with an applicable motor vehicle safety standard.
Because your inquiry relates to school buses, State laws and regulations would determine any obligations school bus owners and operators have regarding on-road use of school buses. Additionally, the Federal Motor Carrier Safety Administration (FMCSA) may regulate some school buses that are used for commercial purposes. I can offer an opinion only on matters within NHTSAs regulatory authority in this case, NHTSAs new vehicle standards. I cannot opine on the applicability of State laws or how NHTSAs new vehicle regulations relate to States or FMCSAs regulation of in-service vehicles.
As set forth in 49 CFR section 567.4, motor vehicle manufacturers are required to affix to each vehicle a permanent label certifying compliance with the FMVSSs. Among the statements required by paragraph (g) of that section to be on the label is the vehicles GVWR and the gross axle weight rating (GAWR), in pounds, for each axle. That label identifies the tire size designation associated with the GAWR.
FMVSS No. 120 requires that motor vehicles with a GVWR over 10,000 pounds must be equipped with tires that meet the requirements of FMVSS Nos. 109, 119, or 139 (all of which provide standards for tires) that are identified as suitable for that rim, as listed in the publication of an approved tire organization or as provided to NHTSA by the tire manufacturer. As you note in your letter, paragraph S5.1.2 of FMVSS No. 120 requires that the sum of the maximum load ratings of the tires fitted to an axle shall not be less than the GAWR of the axle system as specified in the certification label. Vehicles that cannot attain a speed of 50 mph (80 km/h) are excepted from this requirement.
In your request, you cite the example of a school bus with a certification label stating that the vehicles GVWR is 31,000 pounds. The GAWR of the front axle is listed as 12,000 pounds with size 11R22.5(G) tires, and the GAWR of the rear axle is listed as 19,000 pounds with 11R22.5(G) tires in a dual configuration. The (G) designation reflects the load range of the tire. You state that the vehicle in question has two 11R22.5(G) tires mounted on the front axle, but has four 11R22.5(F) tires mounted on the rear axle (which you presume were not installed by the original manufacturer). The (F) designation shows that, although the rear tires are the same size as the front tires, they have a lower load carrying capability.
According to information from the Tire and Rim Association Year Book, the maximum load of each 11R22.5(G) tire is 5,840 pounds mounted in dual use at the maximum inflation pressure of 105 psi. The maximum load of each 11R22.5(F) tire is 5,205 pounds mounted in dual use at the maximum inflation pressure of 90 psi.
You note that the total load ratings of the tires fitted to the rear axle is 20,820 pounds (two pairs of tires mounted in dual use rated at 5,205 pounds each), which exceeds the rear axle GAWR of 19,000 pounds. You state that this satisfies the requirement in S5.1.2 of FMVSS No. 120 that the maximum load ratings of the tires fitted to an axle must be at least the GAWR specified on the certification label. Nevertheless, you ask whether the tire size must be exactly what is stated on the certification label or whether it is sufficient to meet the requirement in S5.1.2 of FMVSS No. 120.
As indicated at the beginning of this letter, we will address whether the tires at issue could have been installed on the vehicle at the time of first sale. The answer is yes. The tires, for a vehicle of this type and GVWR, need not be the exact tire size and load range set forth on the vehicles certification label provided: (1) The size of the tire matches the rim mounted on the vehicle and (2) the sum of the maximum load ratings of the tires fitted to an axle is at least the GAWR of the axle system, as required by S5.1.2 of FMVSS No. 120. For example, in the example you raise, it would be acceptable to install the load range F tires on the rear axle of the vehicle because the maximum load carrying capability of the four load range F tires is greater than the GAWR of the axle. We note that S5.1.2 of FMVSS No. 120 considers the circumstance where the size designation of the tires installed on the vehicle does not appear on the certification label. In that event, S5.1.2 requires that the sum of the maximum load ratings of the tires fitted to the axle shall not be less than the lowest GAWR appearing on the label.
Although there would not be issues with compliance with FMVSS No. 120 as a result of installing tires with a lower load rating than the tires listed on the certification label (provided the sum of the load ratings of the tires installed on each axle is at least the axles GAWR), there may be a safety consequence. In the example you gave, the maximum inflation pressure of the same size load range F and load range G tires is different. The load range G tire has a maximum inflation pressure of 105 psi, whereas the load range F tire has a maximum inflation pressure of 90 psi. That is, the maximum inflation pressure of the rear tire is lower than the recommended inflation pressure on the certification label. In order not to operate on overinflated tires, the operator of the vehicle would have to recognize that the rear tires have a lower maximum inflation pressure. The operator would also have to take care to prevent underinflation, which also may pose a risk because underinflation would reduce the tires load carrying capability.
I hope this information is helpful. If you have any further questions, please feel free to contact David Jasinski of my office at (202) 366-2992.
O. Kevin Vincent
Ref: Standard No. 120
 We also refer you to the parenthetical in S5.3.1 and S5.3.2 of FMVSS No. 120, which states that the tire and rim size designation on the tire information label is not necessarily for the tires and rims on the vehicle.