When Tires Becomes Lean and Green

Tires tested:
Bridgestone Ecopia EP100 (Gr& Touring Summer 185/65R15 88H)
Goodyear Assurance ComforTred (Passenger All-Season 185/65R15 86T)
Goodyear Assurance Fuel Max (Passenger All-Season 185/65R15 86T)
Goodyear Integrity (Passenger All-Season 185/65R15 86S)
Michelin Energy Saver A/S (Passenger All-Season 185/65R15 86T)
Michelin HydroEdge with Green X (Passenger All-Season 185/65R15 86T)
Yokohama dB Super E-Spec (Gr& Touring Summer 185/65R15 88H)

Vehicles used:
2009 Toyota Prius

It takes energy to drive a vehicle down the road & part of that energy is converted into power to overcome the vehicle's aerodynamic drag, driveline friction & the tires' rolling resistance. While drivers can't do much about drag & 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 & the environment? The answer is yes.

Tires are made of about 80% rubber by weight & 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 & treads continuously bend & 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 & 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 & 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 & its contribution to vehicle fuel economy is scheduled to become more readily available in the next couple of years thanks to California & Federal legislation.

While tire rolling resistance often has been reduced in the past by sacrificing wet traction & treadwear, the newest high-tech, fuel-efficient tires proclaim to have achieved low rolling resistance without compromising traction & wear through the introduction of new tread compounds & 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 & measured dry & wet traction limits while braking & cornering.

The Cars

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 & 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 & 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.

The Tires

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 & 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 & belts than st&ard passenger tires to prioritize ride comfort & long wear over fuel efficiency.

All of the 185/65R15-sized tires were new, full tread depth tires that covered several tire performance categories.

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 & wet roads, or for drivers living in snowbelt states that switch to winter / snow tires with the annual change of seasons.

Tire Pressures

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 & 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 & when road & air temperatures had reached their highs. As expected, hot tire pressures had risen with use & measured an appropriate 39 psi front & 37 psi rear, ±0.5 psi.

The Real World Road Ride Conditions

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 & 40 mph county roads along with two stop signs & one traffic light every lap. Our drivers encountered moderate vehicle traffic & 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 & sustain the vehicle's speed using cruise control whenever possible. They also came to a complete stop at the stop signs & waited at the traffic light as necessary.

Fuel Economy Results

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 & absolute distances, along with Linear-Logic ScanGauge IIs to monitor how much fuel was consumed. We alternated which car led our convoy every session & rotated the tires from car to car every half day to assure the accuracy of our data & 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 & tire pressure maintenance), driving conditions (consolidating trips & not speeding) along with a route that only exposed our team to moderate traffic & a limited number of stops.

The fuel economy recorded for each tire & its relative position compared to the baseline Integrity tire's average is shown here:

In the case of the Prius, one of the most fuel-efficient cars in America, the difference between our lowest & 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 & 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 & utility vehicles delivering 12-15 mpg.

What We Learned on the Road

Our expressway, state highway & county roads provide a variety of road conditions that include smooth & coarse concrete, as well as new & patched asphalt. This route allows our team to experience noise comfort, ride quality & everyday h&ling, just as most drivers do during their drive to school or work.

The nature of the 2nd generation Prius & 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 & 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 & 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 & the Yokohama dB Super E-Spec tires which felt harsher over nearly all road surfaces, & the Goodyear Integrity, which had a relatively soft ride but felt harsh when it encountered the bigger bumps & 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 & HydroEdge with Green X. All three were rated closely, with similar overall noise levels & 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 & Integrity produced minimal tread noise, but were marked down for impact noise when they encountered larger bumps & patched sections of pavement.

While not focused on h&ling, the personality & 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 & linear feel to the steering. This came as no surprise as these tires were the only H-speed rated, Gr& Touring Summer tires in the test & didn't require heavily siped, all-season tread designs to provide traction in light snow. H&ling of the HydroEdge with Green X was pleasantly close to the first two tires, with its taut steering & connected feel when driving straight ahead. The Energy Saver A/S followed with somewhat slower steering response & just a hint of vagueness to initial steering inputs. The h&ling 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.

What We Learned on the Test Track

Our track test was designed to measure each tire's straight-line & steady-state cornering traction in wet & 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 & vehicle variables from influencing the outcome. The Integrity tires were run first, last & in the middle of each day's sequence to allow us to identify & account for any changes in the test conditions.

Braking

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 & 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.

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.

Cornering

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 & counterclockwise directions & 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.

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.

Conclusion

Tires shouldn't be considered as an afterthought as just round & black. They've always made important contributions to vehicle h&ling, traction & comfort. & while today's newest low rolling resistance tires are designed to be lean & green, this test has shown many of them can enhance vehicle fuel economy without sacrificing traction.

& as you can see from the top-rated tire in each characteristic we evaluated, many of the tires' names essentially identify their intended strengths.

• Fuel Economy: Michelin Energy Saver A/S (Passenger All-Season 185/65R15 86T)
• Ride Comfort: Goodyear Assurance ComforTred (Passenger All-Season 185/65R15 86T)
• Noise Comfort: Goodyear Assurance ComforTred (Passenger All-Season 185/65R15 86T)
• Road H&ling: Yokohama dB Super E-Spec (Gr& Touring Summer 185/65R15 88H)
• Wet Braking: Bridgestone Ecopia EP100 (Gr& Touring Summer 185/65R15 88H)
• Dry Braking: Goodyear Integrity (Passenger All-Season 185/65R15 86S)
• Wet Cornering: Michelin HydroEdge with Green X (Passenger All-Season 185/65R15 86T)
• Dry Cornering: Goodyear Integrity (Passenger All-Season 185/65R15 86S)

While the Goodyear Integrity tire may have offered state-of-the-art low rolling resistance, fuel economy & 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 & 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:

1. What you drive — Select the tire size & load range that matches your car, van or truck & offers appropriate load capacity to carry the weight of the vehicle, occupants & luggage.
2. Where you drive — Select all-season, summer, winter or on-/off-road tires that match the weather conditions & terrain you expect to encounter. You'd probably forget all about a tire's contributions to fuel economy if you can't make it up a snow-covered hill or slide off an icy road driving summer tires in winter conditions.
3. How you drive — Select passenger, touring, performance or light truck tires that will respond to your personal driving style. Basic tire categories help identify different combinations of comfort, h&ling & traction. Start your selection with the most appropriate tire category for you.
4. How efficiently you drive — Select long-wearing &/or low rolling resistance tires that will give you the miles &/or miles per gallon you desire. But never forget that your vehicle choice & driving skill can influence how much fuel your vehicle consumes each year more than the tires you select.

Not all of today's tires are created equal as some roll more easily than others & 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 & worst fuel economy may not seem like a lot, but it represents a real difference in fuel consumed over time. & while these relatively small percentage improvements in fuel economy may not appear significant, we could significantly reduce fuel consumption & 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 & black!

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