It’s time to rethink standard 100-psi tire pressures
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 Summers, 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,” Summers 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.”
Summers’ 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.
— This story has been updated to correct the name of Brad Summers, the shop manager at Liberty Linehaul. Today’s Trucking regrets the error.
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This is a very good article. Absolutely correct.
Since when is Canada tandem 17000 kgs? What ever happened to 18000 or I miss that memo..
Good catch Cathy. Some jurisdictions allow tandem-axle weights of 18,000 kg, so allow even more in certain configurations. The trouble with making blanket statements regarding vehicle weights and dimensions in Canada is there are always exceptions. The weight I used, 17,000 kg, comes from a memorandum of understanding between provinces on “standard” vehicle configurations that would be allowed to operate anywhere in the country. In essence, 17,000 kg is the lowest common denominator. For the record, the tire load on an 18K tandem group (8 tires) is 4,960 lb, which requires at least 85 psi.
You can read more about Canada’s Truck Weights & Dimensions regulations and how they came to be here: https://www.trucknews.com/features/how-canadas-heavy-truck-weights-and-dimensions-evolved/
Good information. Hard to believe most fleets just inflate and forget. I’ve driven trucks with both under inflated tires and over inflated tires. Both handle very differently. Versus a truck with correctly inflated tires.
Reading between the lines, the next obvious step here would be an on board tire pressure system, capable of optimizing tire pressures. Not just keeping them inflated. Low pressure when running empty, and increased tire pressure when running loaded.
Over inflated tires tend not to work so well on snow and ice either. You may recall the old trick of reducing tire pressure to make it up icy hills etc. And over inflation are also known to have significant push while cornering, even on dry road surfaces.
You comment regarding 13,000 lbs on a steer axle when bobtail worries me too. Than means almost no weight on the drives when operating bobtail or running with an empty trailer. I’m very familiar with that problem. When I drove for Harmac on the Esso account long ago, we had these super short wheel base KW’s with a set back front axle and C15 Cats loading up the steers. Empty, they would get stuck on flat ground with 1 cm of snow hooked to and empty petroleum B-train!
No wonder so many new trucks are bloody useless for traction in the snow with so much weight bearing down on the steer axle.
Who’s building this crap? And how are they allowed to make such dangerously unbalanced trucks? Probably explains the endless jack knifes in incremental weather too.
Good post Steve. The heavy steer weight makes sense when loaded, since that’s “revenue/compliance” condition the unit is in, when scrutized by law enforcement. Most states in the US allow a lot more than 12K lbs. on the steers, some allow as much as 21K lbs. That’s what makes it possible to nail your axle weights with more ease. The “crap” the manufacturers build, is specifically requested by the fleets, after careful analysis of risk, functional necessity, and compliance. You’ve been around the block enough to know that trucks are all about compromise. Weather, terrain, ride, maneuverability, and capacity, among countless other variables, and conditions are what go into specing out the piece of crap a driver has to deal with, while driving empty towards his load.
Just one more thing they failed to mention, when they sold us on becoming a “Bigtruck Driver”.
Being a retire BS/FS employee, this was always a conversation I had with my customers.Stone drilling is greater when the tire is new due to 32/32 tread height.I always told my customers a small screwdriver was a great tool to have for drivers when fueling,driver breaks and pre and post inspection.
So very correct…heat, weight speed and pressure those are the biggest factors understanding the conditions you are running the tires in, the loads you’re carrying and then applying the proper compounds , then I would say mechanical and driver
Thanks for this article! I was just talking about tire pressure today! You’ve helped me realize that I’ve been even more ignorant than could’ve imagined. Over the last 40yrs, I’ve witnessed an evolution of explanation of tire ware. I watched my tires suffer from the torture of my own stupidity.
Thanks again for your efforts to enlighten us!