August 12, 2009
It takes energy to drive a vehicle down the road and part of that energy is converted into power to overcome the vehicle's aerodynamic drag, driveline friction and the tires' rolling resistance. While drivers can't do much about drag and friction once they've bought a vehicle, the question is can eco-friendly, low rolling resistance tires make a difference in fuel economy, driving costs and the environment? The answer is yes.
Tires are made of about 80% rubber by weight and rubber is a viscoelastic material that naturally converts some energy into heat every time it's compressed or stretched. Tires resist rolling primarily because their sidewalls and treads continuously bend and stretch as they transition from their loaded (where the weight of the vehicle causes the sidewalls to bulge as the tread footprint flattens against the road) to unloaded profiles.
Tire rolling resistance and its influence on vehicle fuel efficiency has not been widely rated or reported in the past, so little comparative information is available. However, since every gallon of gasoline saved reduces personal fuel costs and America's dependence on oil, as well as releases 20 fewer pounds of CO2 gas into the atmosphere from the tailpipe, tire rolling resistance information and its contribution to vehicle fuel economy is scheduled to become more readily available in the next couple of years thanks to California and Federal legislation.
While tire rolling resistance often has been reduced in the past by sacrificing wet traction and treadwear, the newest high-tech, fuel-efficient tires proclaim to have achieved low rolling resistance without compromising traction and wear through the introduction of new tread compounds and weight optimizing manufacturing processes.
With multiple manufacturers introducing low rolling resistance tires in 2009, the Tire Rack team felt it was time to evaluate the influence these new tires have on vehicle fuel efficiency, as well as confirm if the manufacturers had to sacrifice traction to achieve it. We included a fuel economy evaluation as part of our Real World Road Ride and measured dry and wet traction limits while braking and cornering.
Unfortunately not all of the latest low rolling resistance tires are available in a size appropriate for our normal test cars, so we arranged to get a small fleet of the popular 2009 model year 2nd generation Toyota Prius hybrids for this test. All of the cars had equivalent miles registering on their odometers and were treated to identical conditions while they were in our care to assure accurate results.
In preparation for our test, the test tires were mounted on Prius Original Equipment 15x6.0" alloy wheels and all of the cars received four-wheel alignments using the vehicle's preferred settings to prevent any unwanted tire scrub from influencing vehicle fuel economy.
Our test began with the premise that the Goodyear Integrity tire available in the U.S. for the 2004-2009 Prius would establish the baseline for this test. We then compared it to today's choices, with one end of the range being eco-friendly aftermarket tires now available for hybrid and fuel-efficient compact cars owners. The other end of our range was represented by the Goodyear Assurance ComforTred. Introduced in 2004 before aftermarket fuel-efficient tires became popular, the Assurance ComforTred features a 20% thicker rubber cushion between its tread and belts than standard passenger tires to prioritize ride comfort and long wear over fuel efficiency.
All of the 185/65R15-sized tires were new, full tread depth tires that covered several tire performance categories.
|Bridgestone Ecopia EP100||Grand Touring Summer|
|Goodyear Assurance ComforTred||Passenger All-Season|
|Goodyear Assurance Fuel Max||Passenger All-Season|
|Goodyear Integrity||Passenger All-Season|
|Michelin Energy Saver A/S||Passenger All-Season|
|Michelin HydroEdge with Green X||Passenger All-Season|
|Yokohama dB Super E-Spec||Grand Touring Summer|
All-season tires are appropriate year-round choices for drivers living in geographic areas that might encounter moderate snow. Summer tires are appropriate choices for drivers living in sunbelt areas that experience only dry and wet roads, or for drivers living in snowbelt states that switch to winter / snow tires with the annual change of seasons.
We could have begun with higher than normal cold tire inflation pressures to improve fuel economy, but we didn't want to deviate from the vehicle manufacturer's placard recommendation for this test. Additionally our experience is that while low tire pressures increase tire rolling resistance, higher than recommended tire pressures don't reduce tire rolling resistance to the same degree.
Cold tire pressures were set to the vehicle manufacturer's recommendation of 35 psi front and 33 psi rear early in the morning after the cars were parked outside overnight. We also checked tire pressures late in the afternoon as our team completed their driving and when road and air temperatures had reached their highs. As expected, hot tire pressures had risen with use and measured an appropriate 39 psi front and 37 psi rear, ±0.5 psi.
Our Real World Road Ride consisted of over 550 miles on each tire during multiple laps of a 6.6-mile loop of public roads that combined 65 mph expressway, 55 mph state highway and 40 mph county roads along with two stop signs and one traffic light every lap. Our drivers encountered moderate vehicle traffic and averaged about 40 mph throughout the day.
In order to keep our fuel economy results as accurate as possible, we only conducted our Real World Road Rides in dry conditions. Since tire treads encounter more resistance driving through water than they do through air, rain soaked roads would have introduced a water depth variable that we couldn't have accurately accounted for.
Since we wanted to compare results that typical drivers would experience, our drivers were instructed to maintain the flow of traffic by running at the posted speed limits and sustain the vehicle's speed using cruise control whenever possible. They also came to a complete stop at the stop signs and waited at the traffic light as necessary.
Rather than using the Prius' on-board trip computer that assumes the dimensions of all tires are identical when it computes fuel economy, we used Race Technology DL1 GPS data loggers to record driving speeds and absolute distances, along with Linear-Logic ScanGauge IIs to monitor how much fuel was consumed. We alternated which car led our convoy every session and rotated the tires from car to car every half day to assure the accuracy of our data and neutralize vehicle variables.
The U.S. EPA rates the 2009 Prius with a combined city/highway fuel economy of 46 mpg using regular grade gasoline. The Prius hybrids used for this test returned an average of about 52 mpg with their air conditioners on. We attribute our higher than normal mpg average primarily to vehicle preparation (accurate alignments and tire pressure maintenance), driving conditions (consolidating trips and not speeding) along with a route that only exposed our team to moderate traffic and a limited number of stops.
The fuel economy recorded for each tire and its relative position compared to the baseline Integrity tire's average is shown here:
@ 15,000 Miles
|% vs. Most Efficient|
|Michelin Energy Saver A/S||53.8||278.8||+4.74%|
|Bridgestone Ecopia EP100||53.5||280.4||+4.12%|
|Yokohama dB Super E-Spec||52.8||284.0||+2.81%|
|Goodyear Assurance Fuel Max||51.6||290.7||+0.37%|
|Michelin HydroEdge with Green X||51.1||293.5||-0.59%|
|Goodyear Assurance ComforTred||50.0||300.0||-2.64%|
|* Our evaluation used Linear Logic ScanGauge II automotive computers to record fuel consumption, and Race Technology DL1 data loggers to record true distance travelled. Offset 6% for Prius Summer E10 regular grade fuel.|
In the case of the Prius, one of the most fuel-efficient cars in America, the difference between our lowest and highest recorded miles per gallon would result in an annual difference of about 21 gallons of regular gasoline consumed at a cost of about $52.50 (at $2.50/gallon) for drivers traveling 15,000 miles a year. Multiply this difference by several years of driving and the value of using low rolling resistance tires steadily increases. Additionally, you can also apply similar percentages of improvement in fuel economy to other vehicles. The fuel saving dollar value of eco-friendly, low rolling resistance tires essentially doubles or quadruples when applied to typical cars that deliver 25-30 mpg or light trucks and utility vehicles delivering 12-15 mpg.
Our expressway, state highway and county roads provide a variety of road conditions that include smooth and coarse concrete, as well as new and patched asphalt. This route allows our team to experience noise comfort, ride quality and everyday handling, just as most drivers do during their drive to school or work.
The nature of the 2nd generation Prius and its design characteristics allow a fair amount of exterior noise into the cabin, which proved to mask some of the tire noise, especially at higher speeds. Our team also found the suspension tuning of the Prius to be somewhat soft, resulting in extra suspension movement over the rough sections of our road ride route. In the end we found that tire choice can have a noticeable influence on how the car drives and feels.
As its name promises, the Goodyear Assurance ComforTred provided the most comfortable ride of the group. This tire did a good job controlling jounce and jiggle as the Prius chassis encountered patched concrete expansion joints. The Bridgestone Ecopia EP100 followed, also doing a good job minimizing how much of the road's bumps made their way to the driver. Right behind was the Michelin Energy Saver A/S, which also displayed good ride characteristics, allowing just slightly more of the road's texture to find its way into the cabin. Not far behind was the other Michelin tire in the test, the HydroEdge with Green X. Behind this front group were the Goodyear Assurance Fuel Max and the Yokohama dB Super E-Spec tires which felt harsher over nearly all road surfaces, and the Goodyear Integrity, which had a relatively soft ride but felt harsh when it encountered the bigger bumps and patched sections of roadway.
For overall noise comfort, the ComforTred again led the group with the least amount of tread noise relative to the other tires in the evaluation. Following close behind were the Ecopia EP100, Energy Saver A/S and HydroEdge with Green X. All three were rated closely, with similar overall noise levels and only small differences in any distinct pitches that could be heard at various road speeds. Next was the Fuel Max, which produced a few distinct tones as it cruised along at higher speeds. The dB Super E-Spec and Integrity produced minimal tread noise, but were marked down for impact noise when they encountered larger bumps and patched sections of pavement.
While not focused on handling, the personality and drivability of the Prius can also be affected by what tire is installed. The firm ride of the dB Super E-Spec paid back with a quick, athletic feel as it responded crisply to driver inputs. So, too, did the Ecopia EP100 that displayed a responsive and linear feel to the steering. This came as no surprise as these tires were the only H-speed rated, Grand Touring Summer tires in the test and didn't require heavily siped, all-season tread designs to provide traction in light snow. Handling of the HydroEdge with Green X was pleasantly close to the first two tires, with its taut steering and connected feel when driving straight ahead. The Energy Saver A/S followed with somewhat slower steering response and just a hint of vagueness to initial steering inputs. The handling of the Fuel Max was appropriate for the category, but not overly responsive. Our team found the soft ride of the ComforTred resulted in a less-responsive tire, feeling slow to respond to driver inputs. The Integrity rounded out the group, feeling a bit sluggish when responding to driver inputs.
Our track test was designed to measure each tire's straight-line and steady-state cornering traction in wet and dry conditions. We used our track's in-ground sprinkler system to provide consistent wet conditions on the first track test day, followed by dry conditions on the second.
One driver tested all of the tires back-to-back on one car to eliminate differences in driver skill and vehicle variables from influencing the outcome. The Integrity tires were run first, last and in the middle of each day's sequence to allow us to identify and account for any changes in the test conditions.
We measured ABS-assisted braking distances from 50 to 0 mph to determine each tire's straight-line traction. We ran multiple stops measured by a Vericom VC2000 braking test computer and averaged the results to determine each tire's typical stopping distances.
In wet braking all of the aftermarket tires stopped in distances ranging from 105.2 to 112.5 feet, however the Integrity tire was separate from the pack when it turned in a long 131.8-foot stopping distance.
|Tire||Wet (feet)||% vs. Baseline|
|Bridgestone Ecopia EP100||105.2||+20.2%|
|Goodyear Assurance ComforTred||106.7||+19.0%|
|Michelin Energy Saver A/S||107.2||+18.7%|
|Yokohama dB Super E-Spec||107.6||+18.4%|
|Goodyear Assurance Fuel Max||108.7||+17.5%|
|Michelin HydroEdge with Green X||112.5||+14.6%|
In dry braking, the Integrity tire held a slim advantage ahead of all the aftermarket tires by turning in a 96.7 foot average. All of the aftermarket tires were close, ranging from 97.2 to 101.2 feet.
|Tire||Dry (feet)||% vs. Baseline|
|Yokohama dB Super E-Spec||97.2||-0.5%|
|Michelin HydroEdge with Green X||97.3||-0.6%|
|Michelin Energy Saver A/S||98.3||-1.6%|
|Goodyear Assurance Fuel Max||99.1||-2.5%|
|Bridgestone Ecopia EP100||101.2||-4.5%|
|Goodyear Assurance ComforTred||101.2||-4.6%|
We measured how fast the Prius could be driven around a 200-foot diameter circle to determine each tire's cornering traction. We timed both clockwise and counterclockwise directions and calculated average cornering values in g-forces of lateral acceleration, with higher numbers indicating better traction.
In wet cornering the aftermarket tires again led the pack, generating average g-forces of 0.677 to 0.638 g. However our wet cornering test again saw the Integrity tire separate itself from the pack with its low 0.601 g of cornering capability. In addition to delivering low g-forces, great patience was required on the part of the driver for the Integrity tire to recover grip every time it exceeded its limit.
|Tire||Wet (g-force)||% vs. Baseline|
|Michelin HydroEdge with Green X||0.677||+12.6%|
|Yokohama dB Super E-Spec||0.670||+11.5%|
|Bridgestone Ecopia EP100||0.661||+10.0%|
|Goodyear Assurance Fuel Max||0.653||+8.7%|
|Goodyear Assurance ComforTred||0.641||+6.7%|
|Michelin Energy Saver A/S||0.638||+6.2%|
In dry cornering like in dry braking, the Integrity tire held a slim advantage ahead of all the aftermarket tires by turning in an 0.804 g average. All of the aftermarket tires were close, ranging from 0.798 g to 0.743 g.
|Tire||Dry (g-force)||% vs. Baseline|
|Yokohama dB Super E-Spec||0.798||-0.7%|
|Michelin Energy Saver A/S||0.795||-1.1%|
|Michelin HydroEdge with Green X||0.795||-1.1%|
|Goodyear Assurance Fuel Max||0.775||-3.6%|
|Bridgestone Ecopia EP100||0.769||-4.4%|
|Goodyear Assurance ComforTred||0.743||-7.6%|
Tires shouldn't be considered as an afterthought as just round and black. They've always made important contributions to vehicle handling, traction and comfort. And while today's newest low rolling resistance tires are designed to be lean and green, this test has shown many of them can enhance vehicle fuel economy without sacrificing traction.
And as you can see from the top-rated tire in each characteristic we evaluated, many of the tires' names essentially identify their intended strengths.
While the Goodyear Integrity tire may have offered state-of-the-art low rolling resistance, fuel economy and traction when it was first developed, its continuing O.E. use essentially froze its specifications. That prevented it from adopting advances in tire design, raw materials and manufacturing methods developed in recent years that have been applied to the newest generation of eco-friendly, low rolling resistance aftermarket tires in this test.
Tire Rack recommends drivers select tires based on their personal needs. Get tires that match:
Not all of today's tires are created equal as some roll more easily than others and consume less energy. This means vehicles equipped with lower rolling resistance tires can achieve better fuel economy than if fitted with higher rolling resistance tires.
The difference between our best and worst fuel economy may not seem like a lot, but it represents a real difference in fuel consumed over time. And while these relatively small percentage improvements in fuel economy may not appear significant, we could significantly reduce fuel consumption and environmental impact if every one of America's 250 million light duty vehicles improved its fuel economy by just a few percent.
While tires have always played an important role for all types of drivers, the manufacturers have just successfully added some more green to the traditional round and black!
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