The rubber hits the regulatory road with the Phase 2 greenhouse gas (GHG) emissions rules crafted by the Environmental Protection Agency and National Highway Traffic Safety Administration. The future-focused truck fuel-efficiency standards have forced OEMs and their suppliers to review all of the components and parts that compose a tractor-trailer—and that includes tires; theoretically, all 18 of them.
In fact, tires account for one-third of a tractor-trailer’s fuel usage (some 2 mpg) at highway speed, with rolling resistance accounting for 35% of the fuel consumed by a big rig, notes Sharon Cowart, director of product marketing for Michelin Truck Tires.
“Keep in mind that for long-haul fleets, fuel costs represent the single highest non-payroll operating expense—and tire rolling resistance accounts for approximately one-third of those fuel costs,” Cowart says. “Thus, the lower the rolling resistance, the less fuel consumed. In fact, a 3% reduction in rolling resistance translates into a 1% fuel savings.”
The North American Council for Freight Efficiency (NACFE) studied the relationship between tire rolling resistance and fuel savings in a Confidence Report released in August 2015. It found that the lower the rolling resistance of a truck or trailer tire, the greater the fuel savings possible—even when the wear rate of a low rolling resistance (LRR) tire is faster than that of “standard” models.
“Tires have a much bigger impact on overall fuel economy than many believe,” notes Mike Roeth, NACFE’s executive director, emphasizing that the cost of the fuel consumed as it relates to tires can be five times greater than the initial purchase price of the tire.
While the typical upfront price of a tire is about 4¢ per mile, given the range of rolling resistance among dual tires, NACFE’s research indicated that tires could actually account for anywhere from 14¢ to 28¢ per mile in fuel costs.
Roeth is quick to admit that “tires are also a tough problem.” They are a major expense and need long life, good reliability and durability, and the tire casing must maintain the capability to be retreaded for a second or even third service life. “Trying to balance all of that with low rolling resistance is not easy,” he stresses.
Trade-offs
And therein lies the real dilemma when it comes to truck and trailer tires versus the Phase 2 GHG rules, explains Brian Buckham, general manager-product marketing for Goodyear.
“Trade-offs have always been part of tire design; the key is to do everything you can to minimize these trade-offs,” he says. “When designing truck tires, we believe it is critically important to achieve the correct balance between fuel efficiency, traction and tread wear,” something Goodyear refers to as “the performance triangle.”
“It’s important to note that low rolling resistance is just one performance benefit that long-haul and vocational fleets demand,” Buckham says. “Increased miles to removal, often achieved through the presence of more wearable rubber; cut, chip and tear resistance; excellent traction; and a premium-quality casing for enhanced retreadability are important needs as well.”
In the past, when one performance benefit in a tire was optimized, such as low rolling resistance, other benefits were reduced. “This dynamic is much less prevalent these days,” he stresses.
Buckham notes that the tire and tread area generates rolling resistance in two ways: via a process called deformation and through rubber compound properties.
“Reductions in rolling resistance can be achieved by reducing the tread block’s height, by stabilizing the tread elements using higher angle drafts, by designing the tread elements to more rigid shapes, or by increasing the surface area of the tread block to increase its stiffness,” he says.
Rubber compound properties are critical as well, Buckham points out.
“Before a truck tire goes to market, the rolling resistance properties of its compounds are evaluated by measuring a dynamic called rebound,” he explains. “Think of bouncing a rubber ball on the ground and then measuring how high it bounces. The higher the ball bounces, the less energy it has lost in its collision, or deformation, with the ground. That’s why compounds with good rebound properties tend to exhibit lower rolling resistance.”
On a typical 6x4 tractor-trailer combination with 18 tires, “eight dual drive tires will contribute about 45% of the total tire rolling resistance level, where eight dual trailer tires contribute about 40% and the two steer tires will contribute 15%,” Buckham notes.
“Drive tires contribute more because they have deep treads as compared to shallower trailer tires,” he adds. “It stands to reason that reducing the rolling resistance of the drive and trailer tires would contribute more to improving fuel economy of a tractor-trailer combination because of the number of tires—16 in all—involved.”
Michelin’s Cowart points out, however, that just as there are many factors in designing a tire, there are also many factors to consider in selecting the right tire for a fleet.
“Selecting the right tire for the right application is of prime importance,” she stresses. “There needs to be a consideration of the operating environment and an understanding of how the tire can contribute to the success of the fleet. That’s why the tire’s performance must sync with the nature of the operations, to the drivers and their habits, driving locations, maintenance habits, budget, and other priorities such as fuel costs.”
Calculating costs
To that end, the best cost per mile or best cost of ownership will dictate what to select, Cowart explains.
“Fleets should work with their tire manufacturer and/or dealer to make the selection based on their needs and budget, and rolling resistance and fuel economy are certainly factors in the decision,” she says.
When it comes to managing the total cost of tires and GHG rules, Rick Phillips, vice president of sales for Yokohama Tires, stresses something else: tire maintenance.
“Maintenance plays a huge role [in fuel economy], and it starts with air pressure,” he says. “Maintaining proper air pressure in the tires not only ensures you can safely carry the load, but it also provides lower rolling resistance and better fuel efficiency. Poor tire maintenance will more than offset the value of fuel-efficient tires.”
In Yokohama’s calculations—and it varies from operation to operation— tires generally account for about 13% of the vehicle’s fuel consumption at highway speeds.
“In other words, 13¢ of every dollar spent on diesel fuel is used to overcome the rolling resistance from the vehicle’s tires,” Phillips says. “Wind resistance becomes more of a factor at higher speeds, but tires still play a critical role in overall fuel efficiency.”
According to Yokohama, steer tires contribute about 15% to the overall rolling resistance of a tractor-trailer, drive tires about 50%, and trailer tires about 35%—with the fuel economy losses due to underinflation commensurate with those percentages.
“Specifically, trailer tires can become a bigger drag on fuel efficiency if they are underinflated,” Phillips notes. “This happens quite frequently because the trailer tire position is usually the most neglected position on the vehicle.”
Maintenance is important
Gregory Kidd, sales engineering manager at Bridgestone, echoes that viewpoint, noting that proper tire use and maintenance go hand-in-hand with achieving the best tire performance.
“But this practice often is overlooked by commercial fleets,” he says. “By maintaining proper cold inflation pressures, selecting the right tires for the job (including the proper tire size and load capacity), and inspecting tires frequently for damage (such as cuts, cracks, bulges and penetrations), fleets can protect their investment and ensure premium tire performance.”
Goodyear’s Buckham agrees. “Air pressure maintenance is the foundation of any effective tire maintenance program. In fact, maintaining correct air pressure is the single most effective maintenance practice that a fleet can employ to impact fuel economy,” he stresses.
“Underinflation, in particular, hurts truck fuel economy. Underinflated tires increase drag resistance, requiring engines to work harder to maintain a constant speed,” Buckham notes. “Maintenance managers should check inflation pressures at least once a week and more frequently if possible.”
Truck tire misalignment can negatively impact fuel efficiency, too.
“When the tractor and trailer are not tracking parallel to the direction of travel, aerodynamic drag will increase, as will lateral forces,” Buckham says. “And just as when tires are underinflated, the tractor’s engine will be forced to work harder to maintain speed and burn more fuel in the process.”
And don’t forget about tractor-trailer speed and its impact on rolling resistance and fuel economy, emphasizes Bridgestone’s Kidd.
“Operating speed is the number-one factor in fuel consumption and has a direct impact on tire efficiency and rolling resistance,” he says. “At operating speeds of 55 mph, tire rolling resistance and air resistance equally consume roughly 33% of usable energy. But increasing operating speeds from 55 to 75 mph can increase fuel consumption by 39% while cutting the effectiveness of fuel-efficient tires by 27%.”
Kidd notes, though, that rolling resistance and fuel economy are influenced by many factors besides speed, including load, inflation pressure, tread pattern, tread wear, tire design, and tire construction.
“Tires should be an important part of every fleet’s fuel economy strategy but also shouldn’t be the only thing they are considering,” he stresses. “Tire rolling resistance can account for 24 to 33% of a vehicle’s efficiency loss. In general, the contribution of the tires on any given axle to overall vehicle fuel efficiency is roughly determined by the amount of load on that axle.
“Selecting the right tire for your application should be based on many factors, not just fuel economy,” Kidd concludes.
How far should you go?
The life expectancy of a tire is a controversial subject with no real black or white answer, but there is agreement as to when a tire is at its fuel-efficient best.
“The best fuel economy a tire can get is right before you pull it from use,” notes Roeth. “Now, you should not use a tire to the point where it is unsafe, but you should make sure you use all the tread. That’s also why the more life a tire has, the more fuel you can save with it.”
Yet, how long is too long? With most truck tire casings also inherently designed to be retreaded at least once, if not more, over a lifetime, is there a point where a tire should simply be discarded because of its age?
“The general industry consensus is 10 years maximum ... after the date of manufacture,” notes Kevin Rohlwing, senior vice president of training for the Tire Industry Assn. “That being said, there is no official guideline that would prevent a company from extending that service life recommendation. If there are no signs of oxidation—also known as dry rot—and the tires appear to be in good condition, then they should be serviceable regardless of age.”
However, adopting a service life recommendation that extends beyond 10 years comes with increased risk. Rohlwing emphasizes that while tire age is a factor, it is not the sole factor determining service life. “As long as the fleet has some data to support its position and knows that operating tires over 10 years old comes with increased risk, then it would be an informed decision that should be defensible in the event of an accident,” he says.
Adding up the numbers
As you read through this article, you may notice that the numbers and percentages cited by the various tire companies are different, sometimes significantly.
That’s not a surprise to industry observers like NACFE's Roeth, whose group conducted extensive research into low rolling resistance (LRR) tires last year. The group has been trying to find ways for fleets to make “straight-up” calculations regarding how to balance the cost of certain tires against the potential fuel economy benefits they can purportedly deliver.
“We think transparency is important, but we don’t have it,” Roeth explains. “Each [tire] brand has its own calculations; there are criteria with data behind it, but it is not public data. That’s why we need the truck and tire manufacturers to explain their numbers better.”
He notes that while the price tag for LRR tires is “all over the map,” “standard” tire models generally cost more. NACFE research concluded that the price premium for LRR models “is usually minor; it’s not huge.”
Still, when trying to figure out if certain fuel-saving tire models are worth the investment, fleets lack a solid measuring stick, says Roeth. “For example, the bar for SmartWay tires is set very low. It is just a fuel economy metric that can be met even by a crappy tire,” he emphasizes. “You can buy a fuel economy tire and end up getting horrendous wear. We’ve heard a lot of horror stories, so you need to do a deeper dive on the tires you are considering using.”
Rohlwing made a similar point a couple of years ago on the tenth anniversary of SmartWay, noting that although the partnership’s efforts saved 120.7 million barrels of oil over a decade—some $16.8 billion worth of fuel savings—that amounts to less than a week’s worth of oil, as the U.S. consumes roughly 19 million barrels of oil per day.
“As to truck tires, SmartWay has created a new market for products that are designed to reduce rolling resistance and improve fuel economy,” he says. “And anyone looking for proof can check the EPA’s website and find 68 tire brands that cover more than 250 models of truck tires that are approved along with 12 retread brands and 38 tread designs.”
But how many of those tires are verified as the result of advanced rubber compounding and/or tread design, and how many rely mostly on shallow tread depth? “When 68 different brands can achieve the coveted SmartWay designation, it’s either a testament to manufacturing or a relatively easy target,” Rohlwing notes.
Despite the EPA’s claim that LRR tires can reduce fuel consumption by 3%, he stresses that fuel savings from tires “is a much more complicated exercise to complete” with many factors beyond rolling resistance.
