Truck tire speed ratings are a guideline, but pressure is the key
A recent investigation by the Associated Press calls into question the safety of commercial truck tires as highway speeds across the country increase. But the AP’s report doesn’t reference a common and accepted practice in the trucking industry: increasing tire pressure to compensate for heavier loads and higher speeds.
Jim Park, equipment editor for Heavy Duty Trucking (HDT), a sister publication of Modern Tire Dealer, is HDT’s go-to tire editor. The AP’s report is based on what Park acknowledges is true – most truck tires are rated for 75 mph. But it neglected to mention what he calls a “rule of thumb” for truckers: “Add 5 psi to their normal inflation pressure for every 5 mph over the rated speed they plan to travel.”
Focused on the 75 mph limit for tires, the AP pointed to 14 states, most of them west of the Mississippi River, that have speed limits of 75 or 80 mph. Parts of Texas have posted 85 mph limits. The AP reports, “Some of those states acted without consulting the tire industry.” The AP discovered the disconnect between highway speed limits and commercial tire speed ratings while covering the government’s investigation of truck tire failures.
The National Highway Traffic Safety Administration determined that truck operators were at fault in blowouts involving certain Michelin tires because in all 16 cases the operators were exceeding the 75 mph rating.
Park, from HDT, says, “There’s nothing particularly wrong with going 75 or even 85, if the proper inflation pressure is observed, even though the tire is rated for only 75. That’s not a fall-off-the-cliff number, just a guideline.
“The problem you will eventually run up against is the max cold inflation pressure rating for the tire and the wheel – usually 120 psi for a load range tire and slightly higher for a load range H.”
As for one of the remedies suggested by the AP – having tire manufacturers make tires with higher speed ratings, Park basically says fat chance.
“It won’t happen. It’s all about fuel economy today so that’s where the R&D dollars are going,” Park says.
For more specifics about the importance of commercial tire pressure, read the latest story on the topic from HDT, from December 2014: Are you putting enough air in your steer tires?
Find the AP’s report here: Big rigs often go faster than tires can handle.