Truck speeds above 75 mpg can compromise tires, but that's less likely to happen if they are properly maintained.  Photo by Jim Park

Truck speeds above 75 mpg can compromise tires, but that's less likely to happen if they are properly maintained. Photo by Jim Park

Is there a speed limit for truck tires? Are truckers wantonly violating those limits and putting the safety of the motoring public at risk? If you read a recent Associated Press story titled "Big Rigs Often Go Faster than Tires Can Handle," you might come to that conclusion. And while there is some truth to the story, it underplays a critical point related to truck speeds and tires: inflation pressure.

The story, published at the end of March, was an AP wire story, so it probably ran in dozens of local newspapers. It suggests truckers often run their trucks at speeds above which the tires are rated. Further, it questions states' decision to raise speed limits on some highways beyond the speed rating of most truck tires. In particular, it calls out 14 states that have speed limits of 75 mph or higher – even as high as 85 mph on a certain highway in Texas.

Do these elevated speed limits really pose a tire-related threat to safety? You can almost hear the pencils scratching away on yellow legal pads around the country. It could be fertile ground for the ambulance chasers. Yes, there is some legitimate concern that high speeds can create safety issues for truck tires. But know this: It's not the tire per se that is the problem – it's the maintenance of the tire, namely how rigorously its inflation pressure is maintained.

As mainstream press truck stories go, the AP article has a predictable bent. The story quotes National Highway Traffic Safety Administration figures that show between 2009 and 2013, there were just over 14,000 fatal crashes in the U.S. involving heavy trucks and buses, which killed almost 16,000 people. Then it notes that tires were a factor in 198 of those crashes and 223 deaths.

In a separate document, Advocates for Highway and Auto Safety cites NHTSA figures estimating crashes caused by large truck tires are responsible for 80 fatalities and 611 injuries annually.

Note, the statistics say "tires were a factor," not the cause, and they don't offer specifics on how tires might have been implicated. It's my guess that speed-related failures would not be significantly represented in those figures. Inflation-related problems, I believe, would account for a larger part of the problem.

Inflating for the load

It's no big secret within the industry that inflation maintenance is weak. Numerous surveys have shown that maintenance of truck tires is poor, with a recent TMC study finding only 44% of tires to be within +/- 5 psi of target pressure. The US DOT came up with similar results in a roadside study conducted several years ago. The recent AP article notes that as part of a separate investigation involving some Michelin tires, "[NHTSA] tested trucks and surveyed over a dozen drivers in Pennsylvania, finding that more than half had overburdened tires because of heavy loads or low air pressure. Sixty percent of drivers didn't know the proper inflation pressure for the trucks they were driving."

While a dozen drivers hardly constitutes a statistically valid number (remember, it's the mainstream press doing the reporting here), I'm not surprised that 60% of drivers didn't know what the proper inflation pressure is.

Back in December, I wrote about inflation pressure in steer tires. I received more than the usual number of comments, indicating that readers were unaware of the elevated pressures required for some steer tires under maximum load (often as high as 120 psi). Clearly, running 100 or 105 psi on some steer tires is inadequate, but hey, who knew? Who ever reads the instructions?

Steer tires typically runs at about 93% of their load capacity, drive tires are around 68%.  Photo by Jim Park

Steer tires typically runs at about 93% of their load capacity, drive tires are around 68%. Photo by Jim Park

Briefly, it's the air inside the tire that supports the weight, not the tire itself. The volume of air inside an 11R 22.5 is larger than a 295/75R22.5, therefore the latter tire requires a higher pressure to support the load. The load range of the tire matters too, as does the actual load on the tire. A quick look at any manufacturer's Load and Inflation tables will show, for example, that an 11R22.5 tire on an axle rated and loaded to 13,200 pounds requires 120 psi. If you're running only 100 or 105, the tire is overloaded/underinflated. To the tire, it's the same thing. It will suffer the same consequences as a result.

Inflating for Speed

Speed is not a critical consideration in tire inflation unless you plan on driving fast. With few exceptions, almost all North American on-highway truck tires are rated for 75 mph. If you abide by the cold inflation pressure requirements for the load on the tires and maintain that pressure rigorously, you won't have any issues with speed.

Some tire manufacturers offer a few words on speed and inflation in their databooks, but the references are usually vague, such as this example:

European ISO sidewall markings can be found on some North American truck tires, but they are not required.

European ISO sidewall markings can be found on some North American truck tires, but they are not required.

"The Tire and Rim Association permits tire load increases, often with increased inflation pressure, for Truck, Bus and Light Truck tires used on improved surfaces at reduced operating speeds. [We] do not condone or recommend operating speeds above posted limits. Rim and wheel manufacturers mark their products with a maximum load and inflation. This applies regardless of operating speed. The rim/wheel manufacturer must be contacted to determine if any deviation is permitted in the marked maximum load and inflation capacity of the rim or wheel at the operating condition in question."

You probably won't find any speed ratings on tires exclusive to North America unless they are speed restricted to 55 mph or less. Tires sold in other markets as well as ours may have markings. European regulators require the sidewall of the tire to be marked with an ISO Load Index and Speed Index. It's usually displayed in a coded number that can't be readily translated into real-world values unless you know the code. Such a rating might appear as "144/142 L" or something similar. A tire bearing such a rating would be suitable for 6,175 lbs single/5,840 lbs dual at a speed of 75 mph.

ISO tire weight ratings are coded, so not easy to interpret at first glance.

ISO tire weight ratings are coded, so not easy to interpret at first glance.

Some North American tires have those ISO LI/SSY markings, but they are not required here under the Code of Federal Regulations addressing truck tires, CMVSS 119. While most commercial tires manufactured by the top suppliers are rated for 75 mph, some tires, particularly those from less-well-known brands, may not be. You would have to consult the tire maker's data book to find out for sure.

In the past, the ATA's Technology & Maintenance Council and the Tire and Rim Association offered recommendations for adding extra inflation to tires or reducing their load if the operator planned to drive at speeds above 65 mph. However, since most tires are now designed for 75-mph operation, many of those recommendations have disappeared. The recommendations were no longer necessary, but some examples remain, and they are probably still valid any time elevated speeds are considered.

One current databook suggests adding 5 psi to the recommended minimum inflation pressure for the indicated load for speeds between 66 and 70 mph, and an additional 5 psi for speed between 71 and 75 mph – or 10 psi above the recommended pressure. Conversely, it suggests reducing the load on the tire by 4% in the former case and 12% in the latter. Not all tire manufacturers offer similar recommendations any longer, but it probably won't hurt to add extra air to the tires if you plan to run above 75 mph.

The ISO Speed index translated. Most North American tires would have an 'L' rating.

The ISO Speed index translated. Most North American tires would have an 'L' rating.

Where you will eventually run into difficulty is in exceeding the maximum cold inflation pressure for the tire (stamped on the sidewall) or the wheel (embossed inside the wheel).

It's all about inflation

From the tire's point of view, under-inflated and over-loaded are the same thing. In either case, there is not enough air in the tire to support the load. Again, from the tire's perspective, the outcome will be the same: The excessive heat generated from the additional internal friction created by the greater flexing of the casing as a given point on the tire rotates through the contact area will eventually compromise the tire.

The higher rotational speed also brings about changes to the shape of the tire, in particular the tread face. Centrifugal force throws the tread-face outward, which changes the contact patch, creates irregular wear and can compromise traction. Recent research has shown this could be more of an issue with wide-base single tires, with their wide and relatively heavy tread-face and low aspect ratio.

So, is it safe to drive at speeds over 75 mph? It's dumb, for a whole lot of reasons, but it's probably not dangerous as far as your tires go – provided tires are rated for 75 mph and are properly inflated. (Prolonged operation at elevated speeds, especially in high-ambient-temperature environments, will not do the tires any favors, mind you.)

As for the AP story's contentions that high speed limits are putting additional operational stress on tires, that's not an incorrect assumption, but urging tire manufacturers to build tires rated for 85 mph is probably not the answer. It's all about fuel economy today, so that's where the tire makers' R&D dollars are going.

Two well-informed tire people I spoke to on background for this story told me that operating at speeds in excess of 75 won't do any major harm to a tire, provided pressure is maintained to a level appropriate for the load and speed. (Their lawyers, however, would probably be more cautious in their response.)

However, they were also quick to point out that prolonged high-speed, high-temperature operation will cause irregular wear and less than expected tire life.

There's another interesting aspect to the AP story – the assertion from one trucking company owner that his tires were somehow to blame for several accidents resulting from blowouts.

His complaints had apparently prompted a NHTSA investigation into the failures. The investigation was eventually closed because investigators had determined that "exceeding the 75-mph rating was the most likely cause in all 16 complaints examined."

The story provides no details on how fast that operator drove his truck, but anyone familiar with the automobile transport sector would be aware of the loads on auto-transport steer axles. Some steer tires commonly used in that sector, 295/60R 22.5, are rated for 7,390 pounds, single, at 130 psi at 65 mph. That load rating is downgraded to 7,160 at 130 psi when traveling at 75 mph.

I am unaware of the details on the tires in question, but at least one databook clearly indicates a pretty high inflation pressure for a pretty heavy load, and it takes the trouble to downrate the load capacity for that model of tire at higher speeds. NTHSA's investigation suggests the operator had a hand in the failures. Given the industry's generally poor record with inflation pressure maintenance, it would not surprise me if those tires were at something less than the recommended pressure for the application.

So, the takeaway to all this is that speed is a factor in determining inflation pressure. With most on-highway truck tires now rated for 75 mph (speed rating L for the ISO LI/SSY rating), 75 should be a safe speed for tires if the correct pressure is maintained. Speeds above 75 may require additional pressure or reducing the load on the tire.

If someone you know likes to go fast, you might want to pass this story on to them. It could save their tires, or more – the truck or a life – in the event of a blowout.

About the author
Jim Park

Jim Park

Equipment Editor

A truck driver and owner-operator for 20 years before becoming a trucking journalist, Jim Park maintains his commercial driver’s license and brings a real-world perspective to Test Drives, as well as to features about equipment spec’ing and trends, maintenance and drivers. His On the Spot videos bring a new dimension to his trucking reporting. And he's the primary host of the HDT Talks Trucking videocast/podcast.

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