“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.