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What Will Tire Rolling Resistance Testing Prove?

NHTSA’s fuel efficiency label scheme may not matter in not-so-perfect world.

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Labels indicating fuel efficiency (rolling resistance), wet grip and noise are now mandatory on all replacement passenger tires sold in the European Union. The new EU label grades tires on a scale from “A” to “G” for fuel efficiency and wet stopping distance.

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The Energy Act of 2007 required NHTSA to establish a national tire fuel efficiency consumer information program for replacement tires – testing and education on tire fuel efficiency, safety and durability.

In March 2010, NHTSA issued its “final rule” on the Tire Fuel Efficiency Consumer Information Program. The actual regulations are expected to be released within the next 12-18 months.

NHTSA plans to require some type of new tire label showing a grade for fuel efficiency (rolling resistance), safety (wet traction), and durability (treadwear). As you might tell, this is more about tire testing and performance measurement than it is about consumer education.

The fuel efficiency rating will be based on rolling resistance. NHTSA conducted a two-phase study to determine how it would rate rolling resistance. Phase 1 was an evaluation of laboratory test protocols, while Phase 2 examined the effects of tire rolling resistance levels on traction, treadwear and vehicle fuel economy.

Wet traction and treadwear ratings are part of the UTQG Standards already required by federal law and the tire labeling scheme will not change the existing UTQG test methods.

The new label will use different ratings than the present sidewall information, which will supposedly give con-­ s­umers true apples-to-apples information so they can compare one tire to the next. If fuel efficiency is their concern, the grades should let them chose the tire that delivers the lowest rolling resistance. Same with “safety” and “durability.”

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Rolling Resistance Factors
Rolling resistance is the force required to move a loaded tire at a constant speed, on a level road in a straight line; it primarily is caused by the high hysteresis of rubber compounds.

Hysteresis is the characteristic of any material that causes the energy requir­ed to deform the material to be greater than the energy of its recovery. Rubber’s high hysteresis means it bounces back slowly and with more resistance than a material like steel that bounces back faster and more completely.

The combination of hysteresis, the tread’s interaction with the road surface, and cycling of the tire’s internal components as the tire rotates through repeated cycles of deformation and recovery produces rolling resistance.

The tread and its underlying plies are a tire’s heaviest and largest components and create most of its rolling resistance – typically about two-thirds of the total. The sidewall and bead area account for the remaining one-third. Because larger tires contain more rubber and internal components than smaller tires, within one particular tire model line, larger sizes will have more rolling resistance than smaller sizes.

There are two principal standards for measuring rolling resistance: rolling resistance force and the rolling resistance coefficient. Rolling resistance force measures energy loss per unit of distance in pounds of resistance. By comparing rolling resistance force, tires of the same or different sizes can be readily and accurately compared.

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The tire rolling resistance coefficient is calculated by dividing the measured rolling resistance force of a particular size tire by its load rating. Rolling resistance varies with the load on a tire, so tires with different load indexes are tested at different loads.

With rolling resistance coefficient,  larger tires may have a lower rolling resistance coefficient than smaller tires, even though larger tires generally have higher rolling resistance forces. Consequently, rolling resistance coefficients only allow realistic comparisons among tires within a single size.

 The NHTSA test will be based on calculated rolling resistance force using the ISO 28580 Draft International Standard the EU selected for its rolling resistance rating system. This should allow harmonization of the U.S. and European standards and test practices.

RR and Fuel Economy
Fuel economy is determined by a vehicle’s total resistance to movement, including aerodynamic drag, driveline friction, inertia, the grade of the roadway and tire rolling resistance. Typically, tire rolling resistance is only 15% of a vehicle’s total resistance in stop-and-go driving. Driveline friction is the largest component at 45%, overcoming inertia represents 35% and aerodynamic drag 5%.

Once out on the highway in relatively steady speed conditions, tire rolling resistance generally represents about 25% of total rolling resistance while aerodynamic drag rises dramatically to about 60%. Driveline friction is only about 15% of the total and overcoming inertia is not a significant factor. 

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The impact of tire rolling resistance on fuel economy ranges from 4% in city driving to 7% on the highway. Auto manufacturers typically estimate that a 10% reduction in tire rolling resistance will result in a 1% to 2% improvement in vehicle fuel economy. NHTSA found that a 10% decrease in tire rolling resistance resulted in a 1.1% increase in fuel economy.

Vehicle manufacturers have continued to demand low rolling resistance tires as OE to help achieve CAFE standards. Tiremakers have responded with reduced weight tires molded with thinner sidewalls, shallower tread depths, and low rolling resistance tread with silica replacing carbon black.

Does It Really Matter?
How much of an actual difference is a set of these lower rolling resistance tires going to make to the consumer? Consider a situation where low rolling resistance replacement tires have a huge 20% decrease in rolling resistance vs. the existing tires.

We need to multiply the portion of the tires’ influence on overall rolling resistance (15% city and 25% highway) by the 20% decrease in tire rolling resistance to calculate the potential change in miles per gallon.

If the vehicle previously provided 25 mpg in the city and 30 mpg on the highway, the calculated increase in fuel mileage for tires with 20% lower rolling resistance would be 3% (25.75 mpg) in city driving and 5% (31.5 mpg) on the highway. A measurable difference, but there are some hurdles to achieving the calculated savings.

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First, the new tires may not initially provide lower rolling resistance as full-tread tires generate more rolling resistance than worn tires. Because of the reduction in tread mass and hardening of the tread compound, tire rolling resistance usually drops 20% during a tire’s life. While the gradual reduction in rolling resistance and any subtle increase in fuel mileage probably went unnoticed, the installation of new tires (even with 20% lower rolling resistance) is likely to be a break-even and may even cause a decrease in fuel mileage.

Also, new, full-tread passenger car tires are typically 0.5-inch larger in diameter than identical worn-out tires. So the revolutions per mile for the new tire may cause the vehicle’s odometer to understate the actual miles driven by 1% to 2%, and fuel economy would appear to decline a like amount.

Finally, tires branded as the same size may vary in their specifications by manufacturer and model. Passenger car tires sometimes vary by as much as 0.2-inch in diameter, with resulting differences in revolutions per mile. If a tire rolls fewer times per mile than the tire it replaces, the vehicle will actually be traveling farther than indicated by the odometer.

Individually, these factors are probably not significant. However, when added together, a consumer’s new low rolling resistance tires may not produce the expected improvement in fuel economy. If OE low rolling resistance tires were replaced, the new tires may even appear to reduce fuel mileage.

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On the other hand, correct tire pressure may be as important as the tires themselves. A 20% reduction in inflation pressure (from 35 psi to 28 psi) may increase tire rolling resistance 10% or more resulting in a 1% to 2% reduction in fuel economy.

The key seems to be in understanding the numbers. Reductions in tire rolling resistance only reduce a portion of the vehicle’s total rolling resistance.

Valid comparisons between tires can only be made within a particular tire classification (e.g. standard touring vs. standard touring), and while lower rolling resistance tires can enhance fuel economy, the day-to-day difference is not large.  

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