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Interpretation ID: 18361.wkm

Mr. Malcolm Prestage
Legislation & Standards Engineer
Hawtal Whiting Engineering Ltd.
Southam Road
Leamington Spa
Warwickshire
CV31 1FQ, England

Dear Mr. Prestage:

Your electronic mail inquiry addressed to Ms. Donna Gilmore of this agency and with the notation "Please forward to Barry Felrice or Administrator for Part 523" was forwarded to this office for reply. You requested clarification of the phrase "static loaded radius arc" found in 49 Code of Federal Regulations (CFR) 523.2, Definitions.

You also requested clarification of the phrase "and the tire inflated to the manufacturer's recommended pressure," which is part of the definition of "static loaded radius arc" found in 49 CFR 523.2. You asked whether that language refers to the tire inflation pressures recommended by the vehicle manufacturer rather than by the tire manufacturer. Following a telephone conversation with Messrs. John Finneran, safety assurance engineer, and Walter Myers of my staff on February 5, 1999, you sent a telefax to Mr. Myers stating that the tire pressures that you would use are within the tire manufacturers' load ranges, although slightly higher than the tire manufacturer's minimum pressures relative to the loads imposed. You sent a further telefax to Mr. Myers on February 12, 1999 in which you stated that your objective is to ensure that a 4-wheel drive (4WD) vehicle with off-road capability would qualify under Part 523 to be classified as a light truck for fuel economy purposes. You also cited the example of a 195/80R15 tire for which the customary minimum recommended inflation pressure would be 33.4 pounds per square inch (psi). By increasing the inflation pressure to 40.6 psi, the static loaded radius, and therefore the vehicle's ground clearance, would be increased by at least 7 millimeters. The 40.6 psi inflation pressure would then become the minimum pressure recommended on the tire label for that axle.

The term "static loaded radius arc" is defined in 49 CFR 523.2 as:

[A] portion of a circle whose center is the center of a standard tire-rim combination of an automobile and whose radius is the distance from that center to the level surface on which the automobile is standing, measured with the automobile at curb weight, the wheel parallel to the vehicle's longitudinal centerline, and the tire inflated to the manufacturer's recommended pressure.

Although the term "static loaded radius arc" does not appear in Part 523 other than in the definitions section, the radius to which it refers is used to determine vehicle classification for purposes of fuel economy standards.

Note that the definition of "static loaded radius arc" includes the criteria by which the radius is to be determined:

  1. Measured from the center of the tire-rim combination to the level surface on which the vehicle stands;

  2. With the vehicle at curb weight;

  3. With the wheel parallel to the longitudinal centerline of the vehicle; and

  4. With the tire inflated to the manufacturer's recommended pressure.

The definition of "curb weight" in Part 523 refers to the definition of "vehicle curb weight" found in 40 CFR 86.082-2, which reads as follows:

Vehicle curb weight means the actual or the manufacturer's estimated weight of the vehicle in operational status with all standard equipment, and weight of fuel at nominal tank capacity, and the weight of optional equipment computed in accordance with 86.082-24; incomplete light-duty trucks shall have the curb weight specified by the manufacturer.

This agency has stated by interpretation that "curb weight" is calculated by adding the weight of the vehicle with all of its standard equipment, including its maximum capacity of fuel, oil, and coolant and the weights of the air conditioner and optional engine, if the vehicle is so equipped. No other optional items of equipment are included, even if the vehicle is equipped with such options, nor are occupants and cargo included in calculating the curb weight.

Paragraph S4.3 of Federal Motor Vehicle Safety Standard (Standard) No. 110, Tire selection and rims, applicable to passenger cars, and paragraph S5.3 of Standard No. 120, Tire selection and rims for motor vehicles other than passenger cars, require vehicle labeling to include the vehicle manufacturer's recommended cold tire inflation pressure for the tires, such that the sum of the load ratings of the tires on each axle is appropriate for the vehicle's gross axle weight rating.

It is this agency's interpretation, therefore, that the static loaded radius arc is determined by making the measurement prescribed in Part 523 with the vehicle at curb weight and the tires inflated to the recommended inflation pressure specified on the vehicle's label. However, in no case may the inflation pressure used to determine the static loaded radius arc exceed the maximum rated inflation pressure specified on the tire sidewall.

I hope this information is helpful to you. Should you have any further questions or need additional information, feel free to contact Walter Myers of my staff at this address or at (202) 366-2992, or by fax at (202) 366-3820.

Sincerely,
Frank Seales, Jr.
Chief Counsel
ref:523
d.4/21/99