use can cut rolling resistance by 5 percent.^40 Polyamide monofilament have been recently
introduced that improve the tire sidewall stiffness and reduce rolling resistance by about 5
percent. These new materials also contribute to reducing tire weight (by as much as 4 kg/tire),
which provides secondary fuel economy benefits and improved ride.
The rolling resistance values of current OEM tires are not well documented. Anecdotal
evidence from experts states that most normal (i.e. not performance-oriented) tires have RRCs of
0.008 to 0.010 as measured by the Society of Automotive Engineers (WE) method.^41
Performance tires used in luxury and sports cars, and increasingly in high performance versions of
family sedans, use H- or V-rated tires that have RRC values of (SAE) 0.012 to 0.013. Tires for
compact vans have RRC values of 0.008 to 0.009 while four-wheel-drive trucks and sport utilities
feature tires with RRC values (SAE) of 0.012 to 0.014.
Potential for Rolling Resistance ImprovementMost manufacturers OTA interviewed had similar expectations for tire rolling resistance
reduction over the next decade. The expectation was that an overall reduction of 30 percent was
feasible by 2005, resulting in normal tires with an RRC of 0.0065 (if the current average is 0.009).
Most also believed the H-rated or V-rated tires would have similar percentage reductions in
rolling resistance so that they would have RRCs of 0.009 to 0.01 by 2005. Very similar
percentage reductions in RRC for light truck tires were also expected. A 30 percent reduction in
rolling resistance can translate to a 5 percent improvement in fuel economy, if the design is
optimized for the tire. Manufacturers were unwilling (or unable) to estimate additional RRC
reductions in the post-2005 time frame, possibly owing to their unfamiliarity with tire
technologies in the research stage at this time.
These 30 percent reductions are expected to be achieved with virtually no loss in handling
properties or in traction and braking. Manufacturers suggested that some loss in ride quality may
occur because of the higher tire pressure, but this could be offset by suspension improvements or
the use of semiactive suspension systems. However, manufacturers expected noise and tire life to
be somewhat worse than those for current tires. Both of these factors are highly important--noise
may represent a special problem because the improved aerodynamics and, possibly, electric
drivetrains of advanced vehicles will reduce other sources of noise.
An optimistic view for the 2015 time frame suggests that RRC values as low as 0.005 may be
achievable. Such low rolling resistance tires have already been built for electric cars. Auto
manufacturers believe that such tires are not yet commercially acceptable because prototypes have
suffered from losses in handling, traction, and durability. Tire manufacturers have expressed the
view that technological improvements during the next 20 years could minimize these losses, and
an RRC of 0.005 could be a realistic goal for a “normal” tire in 2015, as an average, which implies
that some tires would have even lower RRC values.
(^40) "AramidReinforced Tires," Automotive Engineering, vol. 99, No. 8, August 1990.
(^41) SAE has defined a test procedure formeasuring the RRC of a tire alone against a steel drum. When measuredon the car wheel, brake drag and
frictionassociated with bearings and oil seals increasethe total RRC from the SAE-measured 0.008-0.010 to 0.0105-0.0115.