Tires for GHG2: Increased Fuel Economy, Mileage

The next round of federal greenhouse gas emissions reduction mandates is slated to begin in 2021 for trucks and tractors. The stringency of the reductions will ramp up incrementally in 2024 and again in 2027. The Environmental Protection Agency claims these changes will result in a reduction of 1.1 billion tons of carbon emissions and $170 billion of fuel savings for commercial vehicle owners.

Tires will play a significant role in those plans.

Rolling resistance accounts for approximately a third of fuel costs. The lower the rolling resistance, the less fuel consumed. For example, a 3% reduction in rolling resistance translates into a 1% savings in fuel consumed, or an increase of 0.05 mpg. Consequently, ever-lower-rolling-resistance tires are seen as major contributors to the overall vehicle strategy for carbon emissions reductions.

Unlike the EPA’s Smartway initiative with its essentially voluntary compliance, the GHG Phase 2 rules require truck and possibly trailer manufacturers (see page 43) to meet prescribed emissions reduction thresholds. The manufacturers can choose their own pathway to compliance by building vehicles with certain components that contribute to those mandated reductions while accruing credits for compliant builds. At the risk of oversimplifying things, truck makers can load up a spec with certain engine parameters, such as idle shutdown timers and speed limiters, as well as certain components, such as aero fixtures, fuel-efficient drivetrains, tire pressure management systems, and low-rolling resistance tires.

Each component is assigned a numerical value that truck makers can plug into an algorithm, the Greenhouse Gas Emissions Model (GEM), which calculates the proposed reductions in carbon emissions for that truck. 

“Truck manufacturers do not have to test every truck; they are not testing for actual greenhouse gas emissions,” noted Dennis Johnson, the technical director of the EPA’s climate division assessment center, speaking at  American Trucking Associations’ Technology & Maintenance Council meeting in Raleigh, North Carolina, last fall. “Instead, they use certain inputs to the GEM model as a way to demonstrate compliance with the EPA standards.”

As far as the customer is concerned, there are no compliance requirements other than ensuring the vehicle remains in “as manufactured” condition. Certain compliance documentation is affixed to the vehicle to ensure compliance and for enforcement verification.

The upside for the end user is improved fuel efficiency, and as Johnson asked during his TMC presentation, “How many fleets would choose to burn 10% more fuel than they had to?”

Low-rolling-resistance tires can provide a significant fuel savings. “If the low-rolling-resistance tires were to provide a savings of 3%, that would be 450-600 gallons of fuel for a truck running 120,000 miles annually,” said Gary Schroeder, executive director of Cooper’s truck and bus tire business.

However, while fleets are almost always interested in fuel savings, many also factor other performance criteria into their tire spec. There needs to be some consideration for the operating environment and an understanding of how the tire can contribute to the overall success of the fleet, suggests Bill Walmsley, product category manager at Michelin North America.

“The tire’s performance must sync with the nature of their operations, to their drivers and their habits, driving locations, maintenance habits, budget and other priorities – such as fuel costs,” he said.

A New Age in Tires

Historically, this balancing of priorities has left fleets a little uncertain about their tire choices.

“When rubber compounds started changing back in the 1980s, we could demonstrate fuel savings of 3-5%, but those tires were about 15% more expensive and they ran about 30% fewer miles to removal,” said Patricia Meisenholder, Michelin’s product technical manager, at an S.2 Study Group  session at the TMC fall meeting. “Anyone who had run those tires in wet weather probably called their dealers asking what had happened to wet traction.”

One of the hallmarks of the early-generation low-rolling-resistance tires was significantly thinner tread, perhaps 11/32 or 12/32 of tread depth versus 26/32 or 28/32. With simply less rubber to scrub off the tire over time, tires were pulled with fewer miles. Fleets had a hard time reconciling the fuel savings compared with the increases in tire costs – the LRR tires cost more but didn’t run as far.

Tire manufacturers work to optimize what’s known as the “performance triangle” of rolling resistance, traction and miles to removal. In the past, when one performance benefit was optimized, the remaining sides of the triangle sometimes took a back seat. That dynamic is much less prevalent these days, according to tire makers.

“We are constantly developing technologies and designs to minimize tire performance trade-offs, so fleets can enjoy optimal, all-around tire performance,” said Mahesh Kavaturu, Goodyear commercial technology director. “That said, as fuel efficiency regulations tighten, the need for new truck tires and even retreads that offer lower levels of rolling resistance will continue to grow, which is why we are investing even more resources to develop tires that help reduce fuel consumption while, at the same time, helping to ensure that our products provide the traction that fleets also require.”

The major tire companies are all concerned with traction to a certain degree, and many of the changes required to meet rolling resistance through tread rubber compounding have already been made. Now it’s more about design enhancements to the rest of the tire – the tread design, the sidewalls and the bead area.

“We have levers to pull today we didn’t have back in the’90s,” said Meisenholder. “We have tread-compounding to work with, even dual compounding. There’s tread sculpture optimization, tire architecture and wide-base single tires.”

She said there are lots more opportunities today to adjust and experiment with the 200 different chemicals and components inside a tire, including the elastomers and reinforcing agents in the rubber as well as the belts and structural components of the tire. Each one has some impact on rolling resistance.  

“When fleets are choosing tires, they have to balance all their needs, from wet handling, wear and fuel efficiency,” she added. “The good news is, tire manufacturers are prepared to meet the GHG Phase 2 targets.”