On a scale of one to 10, the impact of tire inflation pressure on overall vehicle fuel efficiency is probably no higher than 1.5. But fleets looking to check all the sustainability boxes in their operation still sweat these small details.

Underinflated tires lead to higher than optimal rolling resistance, with leads in turn to higher fuel consumption. We’ll flesh out the impact of tire rolling resistance on fuel efficiency more fully in another article, but suffice to say, for now, it matters.

Reducing fuel consumption is a step in the decarbonization direction. Every liter of fuel not burned is 2.7 kg less carbon dioxide (CO2) pumped into the atmosphere.  

Even if you’re not laying awake at night worrying about what else you can do to forestall climate change, reducing your spend on fuel and tires might be sufficient motivation for the extra bit of effort needed to maintain optimum tire inflation pressure.

What’s optimum?

That’s a question for the ages. If you refer to the tire makers’ Load & Inflation tables, you’ll find pressures surprisingly lower than what has become the industry’s default inflation pressure of about 100 psi.

The number on the L&I tables reflect the minimum inflation pressure required to support the load the tire carries. The maximum pressure the tire can manage is always stamped on the sidewall; it’s usually 120 psi.

For example, in U.S. operations, where axle weights are limited to 34,000 lb. in a tandem group, the maximum weight each tire in the group of eight can legally carry 4,250 lb. (1,925 kg). The minimum pressure required in this example for most common tire sizes is 75-80 psi.

The exception here is the 10’2” spread. With this arrangement, axle weights can be as high as 20,000 lb. per axle, or 40,000 (18,140 kg) for the pair — they are treated as two single axles for weight calculation purposes. Tire loads can be as high as 5,000 lb. (2,270 kg). The minimum inflation pressure required for most common tire sizes would be 95-100 psi.

Here at home, where a tandem axle group can carry 17,000 kg (37,475 lb.), the maximum legal load per tire is 2,125 kg (4,685 lb.) The L&I tables say the minimum pressure for that load is 90-95 psi.

If you inflate drive and trailer tires to 100 psi, as many fleets do, you have a bit of margin to work with if you only ever run the U.S. Fleets hauling close to gross weight in Canada — or using spread-axle tandem trailers — have to be a bit more vigilant with inflation pressure.

Steer tires, just so you know, often require inflation pressures of up to 120 psi with the 13,200- or 14,300-lb. steer axles in common use now. For example, Michelin says the minimum pressure for that load on a 275/80R/22.5 steer tire is 120 psi. It must be a load-range H tire. An LRG tire won’t take that kind of weight or pressure.

The contact patch

In addition to supporting the weight, inflation pressure affects the shape and size of the contact patch. An underinflated tire will have a larger contact patch, ie., more rubber interacting with the pavement surface. This can cause additional rolling resistance and premature tread wear, usually in the center of the tread.

Overinflated tires, on the other hand, have smaller contact patches, which lessens rolling resistance — but also traction. This also leads to premature tread wear, usually wear along the shoulders of the tread.

Your tire dealer should be able to show you images of correct and incorrect contact patches and explain how right-inflating the tires for the loads they carry can reduce tire wear while optimizing rolling resistance.

There are some who believe overinflating tires will improve fuel efficiency, and thus lower CO2 emissions. There’s some truth to that, but it will come at a cost in excessive tire wear. It’s a balancing act.

Does any of this matter?

While tire blowouts and the associated downtime and repair costs usually get everyone’s attention, it’s the slow degradation in fuel efficiency and tire life that eat you alive. Some 80-85% of blowouts are directly related to running dramatically underinflated tires, by the way.

The U.S. Departments of Energy and Transportation have done a number of extensive studies on tire inflation over the years, and even the Commercial Vehicle Safety Alliance (CVSA) has done some work on tire inflation.

Results of those surveys are a bit discouraging. For example:

• About one out of five tractors were found to have one or more tires underinflated by at least 20 psi. Same for trailers.
• A little more than 3% of the trucks surveyed were found to have four or more tires underinflated by at least 20 psi.
• About 3% of all tractors and trailers surveyed had at least one tire running 50% underinflated.
• Just less than half of the all tractor tires and about one third of all trailer tires inspected were within 5 psi of their target pressure.

With those results in mind, it’s worth remembering that running 10 psi below your chosen inflation pressure (all tires) would yield a 1% reduction in fuel efficiency due to the increased rolling resistance of the softer tire, suggests the North American Council on Freight Efficiency (NACFE).  

On the other hand, tire underinflation leads to approximately 5% to 10% degradation in tread wear for each tire underinflated by 10 psi, and 0.5% to 1.0% increase in fuel consumption for a vehicle running with all tires underinflated by 10 psi.

That same 10 psi underinflation can also lead to a decrease in tread life of 5-10%.

If you don’t track tire wear and inflation pressure carefully, you might not notice the difference week to week, but it would surely appear on the balance sheet at year-end.                   

Scope 1 emissions reporting

Many environmentally forward-thinking shippers are starting to look for ways of reducing GHG emissions from their supply chains, and they are casting their gaze our way. They are looking to see what their carrier partners are doing to reduce CO2 output. Fleet fuel economy is certainly something of interest.

Scope 1 emissions are of particular concern. These are greenhouse gases that an organization emits from sources it owns or controls directly. In the case of a truck fleet, the fuel it consumes.

While your fleets may be doing a lot on the front end, such as spec’ing fuel-efficient equipment, those initiatives might be diluted by a poorly managed tire program that’s not tracking the many small leaks in fuel efficiency.

And don’t overlook that fact that decreased tire life through premature tread wear makes its own contribution to climate change. According to the Tire Retread & Repair Information Bureau (TRIB), it takes about 22 gallons (80 liters) of oil to manufacture a new truck tire.

And according to data collected by Canadian Association of Tire Recycling Agencies (CATRA), approximately 19,246 tons of tires were scrapped in 2021 throughout Canada. Making even a small dent in that number will help reduce all sorts of waste-related pollution, including hundreds of tons of CO2 emissions.

Every PSI really does help.