Rolling resistance is the force required to make tires roll and is a characteristic that affects a tire’s fuel efficiency. NHTSA is proposing both a particular rolling resistance metric and a testing procedure manufacturers must use. The results of the rolling resistance test will determine the tire’s fuel efficiency rating on the new consumer label.

NHTSA seeks comment on the difference between requiring a Rolling Resistance Force (RRF) metric versus a Rolling Resistance Coefficient (RRC) metric. NHTSA proposes to use RRF. Rolling resistance is determined by running a tire under a load on a test wheel (as depicted here). RRF is calculated in pounds of force (lbf) or Newtons (N) at the interface of the tire and drum, or the force at the axle in the direction of travel required to make a loaded tire roll. The pending European Union tire efficiency rating system uses RCC, which is RRF divided by the test load on the tire.

NHTSA believes that RFF is the better metric because rolling resistance changes with the load on the tire, which makes direct comparisons between tires tested at different loads difficult. The agency wanted the metric that would best standardize the new fuel efficiency rating and allow accurate comparison among replacement tires. Because RRF measures the energy consumed by the tire near the normal operating conditions in its intended use, NHTSA things that differences in RRF correlate well to the amount of fuel used.

By contrast, NTSHA believes that RRC represents a compressed range of data not as directly correlated to amount of fuel used. NHTSA compared possible tire choices for three vehicles (a Chevrolet Impala, a Chevrolet Silverado, and a Toyota Carolla) and found that a 10 point improvement in a 0 to 100 rating system based on RRF corresponds to a similar amount of fuel saved, no matter what tire size is selected. However, a 10 point improvement on the RRC scale results in a smaller amount of fuel savings for a small car and a larger amount of fuel savings for a large car.

Because the tire fuel efficiency rating system may fail if it is not intuitive to consumers, NHTSA is concerned that a rating system in which a given increment of rating change could represent different gains in fuel efficiency for different size tires may confuse consumers. Using the RRF metric will allow the agency to present general “rules of thumb” for how a difference on the rating scale translates to increased fuel efficiency.

Related issue for comment: Rolling Resistance Test Procedure