Under the proposed rule, tire manufacturers would be required to submit data to NHTSA on the fuel efficiency, safety, and durability of each covered tire model. (For the proposed testing metrics and procedures, see “Testing Requirements” in the Rule Map.)

NHTSA is seeking comments on its procedure for what constitutes noncompliance with the reporting and rating requirement it intends to establish. It wants to determine a range of tolerance within which a manufacturer’s reported ratings may deviate, without penalty, from the ratings produced by NHTSA’s own testing.  Generally, NHTSA proposes to require that the manufacturer’s rating be either less than or equal to the agency’s rating as determined by the specified testing procedures, or within a specified tolerance range.

Fuel Efficiency
NHTSA proposes a fuel efficiency tolerance of plus or minus 5.5 % of the rolling resistance force (RRF) determined through the specified test procedure.  The agency discovered through its test data that the RRF of a tire varies according to the tire’s load rating.  Although  smaller tires with a smaller load-range index displayed smaller variations than tires with larger load-range indexes, the RRF remained within 5.5% of the respective means.  So, a tolerance of plus or minus 5.5% seems appropriate to account for this source of variance.

NHTSA recognizes that other variables (e.g., different batches of material; different manufacturing dates; variations in testing machines and labs) may cause a variations in measured RRF.  The agency seeks comments and proposals on these various factors.  Proposals should be supported with data.

Safety
With respect to the reported safety (i.e., traction) rating, NHTSA proposes that the adjusted peak coefficients of friction for asphalt and for concrete must each be within 0.06 of the respective peak coefficients determined by the agency.  (See Rating Formula; Safety Testing).   The 0.06 number is based on test data in which peak coefficients of friction for asphalt and concrete were recorded for many tire models, and the average and standard deviations were calculated for each.

As with the fuel efficiency tolerance, NHTSA recognizes that other factors may affect the variation of the peak coefficients.  The agency is seeking comments and proposals on these various factors, and requests that they be supported with data.

Durability
NHTSA proposes a tolerance in the durability (i.e., treadwear) rating of 2.5 mils per 1,000 miles of the Uniform Tire Quality Grading Standards (UTQGS) wear rate.  (See Durability testing).  The agency arrived at the 2.5 number through testing on the ASTM E1136 Course Monitoring Tire, using the UTQGS specifications found in 49 CFR 575.104(e).

As with the fuel efficiency and safety tolerances, there are other factors that may affect the variation of this rating.  The agency is seeking comments and proposals on these various factors, and requests that they be supported with data.

The “safety” rating that will be included on the proposed new consumer information tire label will be based on traction — specifically, a measure of the tire’s ability to stop on wet pavement.

NHTSA proposes to base a tire’s safety rating on the results of test procedures currently required under NHTSA’s  Uniform Tire Quality Grading Standards (UTQGS) tire rating system.  The agency intends to use these existing test procedures because the Energy Independent and Security Act of 2007 requires that the new tire efficiency consumer information rule be finalized by the end of 2009.  However, NHTSA proposes that the UTQGS test results be normalized to a 0 to 100 point scale for display on the new label, to make the safety/wet traction rating comparable to the other two ratings.

In the UTQGS procedure, a tire is tested on both wet asphalt and wet concrete. The tire is attached to the axle of a trailer and towed behind a vehicle at 40 miles per hour. The trailer’s brakes are locked, causing the tire to skid. Sensors on the axle calculate the resulting braking forces, yielding a measurement called the sliding coefficient of friction. This measurement becomes the basis for the tire’s safety rating.  See Rating Forumlas.

Of course, most modern vehicles would not allow such a skid to occur. New cars and trucks are usually equipped with Antilock Braking System (ABS), Electronic Stability Control (ESC), or both. Therefore, a tire’s sliding coefficient of friction may suggest that a tire is less safe — in terms of wet traction — than it may actually be. For this reason, NHTSA is considering basing a tire’s safety rating on its peak, as opposed to its sliding, coefficient of friction. This measurement, also calculated during the UTQGS test procedure, may more accurately represent a tire’s safety when used with a vehicle with ABS, ESC, or both.

A potential drawback of basing a tire’s safety rating on its peak coefficient of friction is that many older vehicles still in use are not equipped with ABS or ESC. NHTSA found that a non-ABS equipped vehicle required a significantly longer wet stopping distance than an ABS equipped vehicle using the same tire. Therefore, a tire safety rating based on peak coefficient of friction could be misleading to owners of older vehicles.

Recognizing the complexities of a single traction rating that will be applied to both older and newer vehicles,  NHTSA is considering basing the safety rating on a composite of four measurements: peak coefficient of friction on wet asphalt, peak coefficient of friction on wet concrete, sliding coefficient of friction on wet asphalt and sliding coefficient of friction on wet concrete.   These four friction values could be weighted equally, or differently.  The agency is looking for comment on the idea of a composite rating, and on how the four elements ought be weighted.

Because asphalt and concrete surfaces can vary from day to day and place to place, NHTSA proposes to require tire manufactures to adjust their testing results by correlating to a “standard reference tire.” These adjustments would be accomplished using the formula described at Rating Formulas.

“Rolling resistance” is the force required to make tires roll and is a characteristic that affects tire fuel efficiency. NHTSA is proposing a specific test procedure by which manufacturers are to measure rolling resistance for the planned new tire label.
Related issue for comment: Rolling Resistance metric.

Rolling resistance represents all the energy losses associated with a tire rolling under a load. In general, rolling resistance is tested in a laboratory by running a tire on a test wheel (as depicted here).  NHTSA is interested in measuring rolling resistance at a constant speed, and therefore it considered only “steady state” testing methods.

NHTSA is looking for the test procedure that will best standardize the fuel efficiency rating and so provide an accurate comparison across replacement tires.   In a study concluded in October 2008, the agency evaluated five existing and draft test procedures, including procedures from both the Society of Automotive Engineers (SAE) (SAE J1269single, SAE J1269multi; SAE J2452) and the International Organization for Standardization (ISO) (ISO 18164:2005E; ISO 28580).  Three of these methods are multi- point tests, two are single point tests.  A multi-point test uses various conditions of speed, pressure and/or load.  A single point test applies a single set of conditions – those mostly likely to be the average conditions in which the tire would operate.

Within a particular test procedure, four methods can be used for measuring the energy loss that is converted to rolling resistance: force method, torque method, power method, and deceleration method. The force and torque methods are most common. In the October 2008 study, one laboratory evaluated all five test procedures on a single “force measurement method” machine.  A second laboratory evaluated one procedure on a “torque measurement method” machine and the other four procedures on a “force measurement method” machine.

The October 2008 study found that each of the five methods reached very similar results: the ranking order for various tires’ rolling resistance was largely identical. Because single-point testing is less expensive and less time consuming, NHTSA is inclined to such a procedure. However, the agency requests comments about the benefits and drawbacks of adopting a single point test.

The specific single-point procedure NHTSA proposes to require is ISO 28580. As between the two single point test methods evaluated, NHTSA prefers ISO 28580 because:

• ISO 28580 specifies a procedure by which to correlate results between laboratories and test equipment. NHTSA found that differences between labs and testing equipment were a significant source of variation;  ISO 28580 is the only procedure with a mechanism for dealing with this variation.  To adopt a different test method, NHTSA would have to develop a procedure to standardize results generated by different laboratories using different test equipment.
• ISO 28580 is the procedure specified in the pending European Union tire labeling program. Therefore manufacturers selling tires in both the US and EU countries would  need to perform only one rolling resistance test.

ISO 28580 uses capped inflation pressure.  NHTSA believes that this (as opposed to regulated inflation pressure) is a more accurate specification to use, but it seeks comment on this point.

NHTSA also seeks comment as to the type of surface — either textured or bare steel — that should be specified for used on the “roadwheel,” the wheel on which the tire being tested will run. The agency is proposing that an 80 grit surface be used, both to prevent slippage and to make test results more repeatable. However, NHTSA would like comments on whether lab correlation procedures could effectively account for differences generated using different roadwheel surfaces.