Tire inflation pressure is a real conversation killer at a party, but it’s something fleets should be talking about with their tire vendors, shop personnel, and drivers. Evidence suggests the set-it-and-forget-it, all-position, one-size-fits-all inflation pressure of 100 psi is not the best way to maximize tire value.
The number is easy to remember, but you may be sacrificing miles-per-32nd for the sake of convenience.
The two fleets we interviewed for this article are running 85 psi and 90 psi in their drive and trailer tires in longhaul Canada-U.S. service operating at an 80,000 lb. gross vehicle weight or less. They report tires running out to 400,000 and 500,000 kilometers before coming off the truck. Drive tires running to 800,000 km are not unheard of in these two fleets.
How does one coax more than half a million kilometers out of a set of drive tires? You pay attention to them, advises Mark Reavely, fleet maintenance manager at Trans-Frt McNamara of Ayr, Ontario. His fleet gets 500,000-575,000 km out of drive tires.
The related tire program includes annual three-axle tractor alignment, annual replacement of the shock absorbers, and regular manual tire pressure checks. No tire pressure monitoring system or automatic tire inflation is involved. He runs 85 psi in his drive tires and 90 or 95 psi in his trailer tires, adjusting the pressures for seasonality.
“We worked closely with our tire vendor to come up with those pressures,” Reavely says. “You don’t need anything close to 100 psi with the weights we run, so we discussed what the optimum pressure should be based on their engineering and experience, and 85 psi seems to be the magic number for our drive tires [Bridgestone M760]. We’re getting really even treadwear straight across, and a better footprint on the road.”
Reavely says he hasn’t lost a drive tire to irregular wear since 2018, and he doesn’t see the stone drilling issues other fleets report with those tires.
Similarly, Brad Sellers, the shop manager at Liberty Linehaul — also of Ayr, Ontario — runs 90 psi in both his drive and trailer tires. He says it’s all about the tire’s footprint or contact patch.
“A flat, square footprint is what you want for even tread wear,” Sellers says. “If you over-inflate the tire, the footprint becomes rounded at the sides and the front and rear. If you see a rounded footprint, you are going to get irregular wear on the edges, which shortens tire life, and you get less traction because the portion of the tire in contact with the road is smaller, especially when you’re lightly loaded.”
Sellers’ fleet target for drive tire life is 450,000 to 500,000 km, and he hardly ever sees tires that deliver less. When they do, it’s usually because of some external factor, such as an alignment problem or a road hazard. He too consults heavily with his tire vendor, and together they worked out the optimum pressure of 90 psi.
“Talk to your vendor and get them to show you pictures of the footprints at various pressure and you can see the difference clearly,” he says. “The tire companies have that information. It can help determine the best inflation pressure for your loads based on the load and inflation tables.”
Dropping drive and trailer tire inflation pressure to something less than 100 psi might seem like heresy in some circles, but there is technically no need to inflate tires to a higher pressure than what’s needed to support the load on the tire. There are drawbacks to both over- and under-inflating drive and trailer tires.
Steer tires are different, they generally need every pound of pressure they can get.
80, 90 or 100 psi?
To understand inflation pressure, you have to understand that it’s not the tire itself that supports the load, but the air inside the tire. The higher the pressure, the heavier the load the tire can support.
The volume of air in the tire matters, too. Tires with a larger internal volume can support greater loads at similar pressures. For example, based on Continental Tire’s load and inflation tables, an 11R24.5 tire at 90 psi can support 5,840 lb. An 11R22.5 tire can support 5,510 lb., while a 275/80R22.5 tire can handle 5,370 lb.
The same table indicates that an 11R22.5 tire at 100 psi in a dual configuration can support 5,625 lb. Now, consider the actual load on a tire. In a dual configuration on a tandem axle in the U.S., where the maximum legal axle group load is 34,000 lb., the maximum load on any one of the eight tires in the tandem group would be just 4,250 lb.
In most of Canada where the tandem group weight limit is 17,000 kg (37,500 lb.), single tire loads in dual configuration will not exceed 4,690 lb. That’s significantly less than the tire’s weight-bearing capacity at 100 psi. In both the U.S. and Canadian examples, the minimum pressure required to support those tire loads on an 11R22.5 tire would be 70 psi.
Load and inflation tables are not marketing brochures. They are technical documents describing the minimum inflation pressure required to support a given load. You’ll find very little variation across tables published by different tire manufactures, or even the Tire and Rim Association’s tables. But the data may be presented in slightly different ways.
Those figures do not represent recommended inflation pressures, either. It’s up to the user to decide what inflation pressure to use as long as it meets or exceeds the minimum. So, if 70 psi is the minimum, what happens to the tire when it’s “over-inflated” by 30 psi?
While it’s not as much of a concern as it once was — and this does vary across tire brands and models — the inflation pressure can influence the size and shape of the tire’s contact patch. An ideal contact patch for a typical drive or trailer tire would be roughly a 10-inch square. Over-inflated tires can exhibit a narrower contact patch, rounded at the front and back, which illustrates that there’s less rubber actually contacting the pavement. You’re more apt to see this in a lightly loaded tire with high inflation pressure.
Edge wear can increase, and traction is compromised, especially when lightly loaded. It also stiffens the tread face, making it more susceptible to stone drilling and punctures.
We aren’t suggesting you reduce your tire pressures to 70 psi, but something less than 100 might provide longer tire life and happier drivers due to improvements in the ride quality. We know of one American fleet that tried 70 psi in its drive tires. The results in terms of ride quality and tire wear were very good, but they kept getting pulled around back at the scales because the inspectors saw higher-than-normal sidewall deflection, and believed the tires to be “run-flats”.
If any of this resonates with you, get your tire vendor on the phone to discuss inflation pressure. Don’t rely on anecdotal evidence or tribal knowledge. Tell them you want to see test results, footprint charts, etc. They should be happy to provide that information if it will make their tire last longer.
When it comes to fuel economy, there may be marginal gains, like 1-1.5%, from running over-inflated tires. But running tires out to 500,000, 600,000 and even 800,000 kilometers with fewer puncture failures will probably offset the fuel saving.
Have that conversation with your tire supplier, but not a party.