NHTSA's standard reference test tire: the 2022 rule that swapped a 14-inch baseline for a 16-inch one

On June 8, 2022, the US National Highway Traffic Safety Administration (NHTSA) published a final rule amending Federal motor vehicle safety standards and consumer information regulations to update the standard reference test tire (SRTT; British spelling: tyre) used in them. The rule, carried at 87 FR 34800 under Docket No. NHTSA-2020-0067, amends 49 CFR parts 571 and 575 to replace a 14-inch SRTT with a 16-inch one as the baseline tire in those standards and regulations.

The SRTT is not a tire consumers buy. It is a reference tire defined in the rule text and incorporated into the Code of Federal Regulations, against which test results under US safety and consumer-information requirements are measured. The 2022 rule matters to anyone who follows tire ratings or braking requirements in the US market because the same reference sits behind several of them.

Three documented roles in the US rule set

NHTSA's rule text describes three ways the SRTT is used in US requirements.

• Surface friction: under the Federal Motor Vehicle Safety Standards (49 CFR part 571), the SRTT is used to verify the surface friction of test surfaces for braking and electronic stability control standards.
• Snow tire determination: it serves as a traction reference for determining whether a tire may be considered a "snow tire" under FMVSS No. 139 (49 CFR 571.139).
• Treadwear grading: it is the course reference tire in NHTSA's Uniform Tire Quality Grading Standards (UTQGS) consumer information program, as part of the base course wear rating (BCWR) determination for treadwear.

How the 14-inch tire entered the rules, and why it changed

NHTSA first incorporated the 14-inch SRTT into the Federal Motor Vehicle Safety Standards in a 1995 rule adopting FMVSS No. 135, the light vehicle braking standard, where the tire is used to determine the friction of the test surface. Testing to determine the consequences of replacing the 14-inch SRTT with the 16-inch SRTT was conducted by entities including the Rubber Association of Canada; the results of that testing, together with NHTSA's own testing, substantially contributed to the agency's August 2021 proposal (NPRM) to make the replacement.

The rulemaking record also states NHTSA's expectation about the effect of the change: with an equivalent level of surface friction determined when evaluating PFC with the 16-inch SRTT in place of the 14-inch SRTT, the agency determined that the change to the standard reference test tire should not change the performance of current tires or vehicles.

What was actually amended

In the regulatory text, NHTSA amended § 571.5, the provision governing material incorporated by reference into part 571: it revised paragraph (a), removed and reserved paragraph (d)(33), and revised paragraphs (d)(34) and (35). Those paragraphs carry the incorporation-by-reference of the SRTT specifications, which is how a tire design defined outside the Federal Register text becomes binding inside the rules.

Incorporation by reference follows a legal procedure that the rule itself sets out: under 5 U.S.C. 552(a)(1), NHTSA must publish the text of its rules in the Federal Register, with the exception that matter reasonably available that cannot be published there may be deemed published when incorporated by reference with the approval of the Director of the Federal Register, under the policy in 1 CFR 51.1(f).

In scope here is the SRTT replacement rule itself: its three uses, its background and the § 571.5 amendments. Treadwear grades for specific tire lines, US tire-testing practice and other NHTSA tire rulemakings, such as the agency's 2019 docket on tire strength, bead unseating and endurance tests, are separate subjects.

Checking the rule yourself

• Read the final rule as published on 06/08/2022 (Federal Register document 2022-12243, 87 FR 34800, Docket No. NHTSA-2020-0067) on federalregister.gov, and compare it with the August 5, 2021 NPRM in the same docket.
• For legal work, verify the text against an official edition on govinfo.gov; the Federal Register site states that those relying on it for legal research should verify their results against an official edition, since its online rendition does not provide legal notice until granted official legal status.
• To see the SRTT references as amended by the 2022 rule, look up the § 571.5 incorporation-by-reference list and the treadwear provisions in part 575; check the eCFR edition you read for any later amendment.