Automatic tire inflation systems are being considered as a fuel-saving technology under the Greenhouse Gas Phase 2 rule. As the proposal currently stands, the Environmental Protection Agency and the National Highway Traffic Safety Administration are prepared to offer compliance credits to any original equipment maker that installs such systems.

Paul Crehan, director of product marketing for Michelin Americas Truck Tires, told HDT the agencies’ decision was based on evidence from a DOT study carried out between 2008 and 2010 with two carriers, Sheetz Petroleum and GFS Food Service.

“DOT found an average of 1.4% improvement in fuel economy due to tire inflation systems and wheel-mounted and valve-stem mounted TPMS (tire pressure monitoring systems) collectively,” Crehan says. “Taking this study as reference, [GHG] Phase 2 has proposed to account for a 1.5% factor for fuel economy if trailer ATIS is installed and 1% if tractor ATIS is installed.”

There’s little debate about the cost effectiveness of trailer ATIS when it comes to reducing the downtime costs associated with flat tires, but how much of a contribution can they realistically make to fuel economy?

Almost everyone is familiar with the adage that 10% under-inflation can cause a 1% degradation (or something to that effect) in fuel economy due to the increased rolling resistance of softer tires.

But what exactly does that mean? Are we talking about one tire out of 18, just the tires on the truck, just the tires on the trailer, or would all the tires have to 10% under-inflated to account for the 1% reduction in fuel economy?

We asked several tire makers that question, and the answer was universal: all the tires need to be 10% under-inflated before the truck sees a 1% drop in fuel economy. But it’s a little more complex than that.

Here’s how Terry Smouter, business development manager for Continental Commercial Vehicle Tires, describes it: “The overall fuel economy of the vehicle is dependent on the axle position of the under-inflated tire. The drive and trailer axles contribute more to the overall fuel economy of a truck than the steer position does.”

There have been multiple studies conducted both in Europe and here at home that show, in general, that tire pressure maintenance isn’t something most fleets are good at.

Smouter cited a European study that shows 12% of fleets are running tires that are under-inflated compared to the fleet’s inflation guidelines.

And Crehan cited a DOT study from 2003 that showed only 50% of tires were within 5 psi of recommended pressure, 1 out of 20 tires was more than 20 psi under-inflated and 20% of dual-tire assemblies have tire pressures that differ by more than 5 psi.

Recommended pressure

But here’s the rub. What exactly is recommended pressure, and how much is it costing fleets in fuel economy if they run tires at less than recommended pressure?

Try as you might, you will find only one official reference to tire pressure anywhere in this land; the tire makers’ load and inflation tables (also available through the Tire Industry Association). The L&IT speak only to the minimum pressure required to support a given load, which isn’t much to go on.

“The load and inflation tables are standards that are consistently maintained by all tire manufacturers and provide the guidelines to which tires can safely carry a given load,” explains Prosser Carnegie, product development manager for Continental Truck Tires, NAFTA. “The key concern as a tire manufacturer is when a tire is being run under the L&IT guideline, since this is approaching an unsafe operating condition. At the minimum pressure, the tire is essentially operating at the threshold of its load carrying capacity.”

It can be concluded than that any tire run below the minimum stated in the L&IT for a given load is technically under-inflated and at risk of related damage, such as tread separation due to high operating temperatures and increased flexing of the sidewalls, which can lead to zipper ruptures.

How, then, do we reconcile a tire running at 90 psi when the L&IT says a minimum of, say, 80 psi is required for the load? Is this under-inflated? 

“That would not be considered an under-inflated tire in the strictest terms when it comes to carrying a given load,” Carnegie says. “But consider a fleet whose standard pressure for a drive tire is 100 psi. The 10% under-inflation is not in relation to the L&IT standards, but 10% under normal operating inflation pressures.”

That might clear up the matter of what defines an under-inflated tire, but there’s still another question to grapple with. Are fleets that choose to run 90 psi in their tires, rather than 100, giving up 1-1.5% in fuel economy?

Possibly, but weighed against other operational concerns involving tire inflation pressure, it might seem like a fair trade-off.

“There are factors outside of pure load carrying capacity that can be optimized with higher inflation pressures,” Carnegie says. “This is something that is tuned based on each individual fleet’s application and tire performance needs.”

Fleets that consistently run lighter loads, for instance, may see some tire wear benefit from running lower inflation pressures, say 85-90 psi rather than 100. The softer tire’s contact patch may be more even, thus contributing to longer wearing tires with less irregular wear.

But under-inflation can create a number of problems as well.

“Under-inflation causes tires to flex more as they roll down the highway and can hurt a truck’s overall fuel efficiency, due to the simple fact that under-inflated tires force a vehicle’s engine to work harder,” says Brian Buckham, general manager for product marketing at Goodyear Commercial Tire Systems.

Peggy Fisher, CEO of TireStamp, a provider of tire pressure monitoring systems, points to an experience she had during her early days at Roadway when radial tires were still relatively new.

“We were getting some very uneven wear on those tires,” Fisher recalls. “Our supplier at the time recommended that we drop pressure from 100 to 85 psi. The only thing that did was noticeably increase fuel consumption. Needless to say, we went back to 100 psi very quickly.”

Is more better?

It’s accepted that running 100 psi rather than the L&IT minimum of 75-80 psi will improve fuel economy by reducing rolling resistance. So why stop there? Why not inflate to 110 or even more?

According to Continental’s Smouter, running 100-110 psi on drive and trailer tires would lead to better fuel efficiency than 75-80 at the maximum legal load, but there are consequences at the high end, too.

“As the inflation pressure increases, there is a potential for more irregular wear and more impact breaks,” he says. “Each fleet needs to find their own ideal operating inflation pressures based on their individual application and needs.”

And that brings us right back to “proper” inflation pressure. The answer to the inflation question really is whatever works for your fleet.

“To gain the benefits of fuel efficiency, tires need to be inflated to the proper pressure for the specific load to get the maximum benefit,” says Michelin’s Crehan. “Tire inflation contributes to more than fuel efficiency. Proper tire pressure contributes to maximum total performance including wear, retreadability, casing endurance and traction.” 

With that said, should fleets worry if their 100-psi tires are found to be at only 90 psi?

“That would indicate something is wrong with the tire,” Fisher says. “It’s either leaking or it hasn’t been maintained for a very long time. If all the tires are 90 psi, they probably have not been serviced in months.”

Expanding on that point, Crehan says tires found to be at 20% less than the recommended inflation target — that is, the fleet’s chosen pressure provided it’s within the minimums prescribed by the L&I tables — must be considered as run flat and removed.

To answer the question of the effectiveness of automatic tire inflation systems, or even tire pressure monitoring systems, if eventually included in the final GHG Phase 2 rule: They will probably help, but they come with issues of their own, including additional maintenance in some cases.

“ATIS will make a difference to fleets that don’t see their trailers very often to maintain them or who just don’t maintain them,” says Fisher.

Given fleet track records with pressure maintenance, maybe EPA and NHTSA will be handing us a benefit we won’t even recognize until we get it.