Feeling the need for speed

May 11, 2015
Increased speeds can increase the chance for tire problems

When the National Maximum Speed Law was passed in 1974 as part of the Emergency Highway Energy Conservation Act, the primary purpose of the national 55 mph speed limit was to reduce gasoline consumption. By the late 1980s, Congress had raised the maximum speed limit to 65 mph and in 1995, the authority for establishing speed limits was returned to each individual state. Since then, maximum highway speeds have ranged from 55 to 85 mph.

While there is a lot of debate regarding the relationship between speed and fuel consumption, most fleets have made the decision for over-the-road drivers by governing trucks to 65 mph. Whether they are motivated by costs or safety, the fact remains that the majority of trucks on the highway are not capable of speeds greater than 65 mph. However, there are still a significant number of trucks that are not governed, so they can travel at speeds much higher than posted limits.

The mainstream press has recently picked up on the fact that with speed limits over 75 mph for trucks and seemingly rising, the speed capability of truck tires has been called into question. In most cases, it’s just another example of investigative reporters trying to “uncover” a safety issue that potentially puts the motoring public in danger. There are only a handful of states with posted truck speed limits over 75 mph, which means the overall risk is minimal.

Even though federal regulations do not require tire manufacturers to label every truck tire with a “speed rating,” every truck tire has one. When the speed is restricted to 55 mph or less, then the manufacturer must mold that restriction on the sidewall. Otherwise, the default speed rating for most highway truck tires is 75 mph. In fact, it’s safe to say that unless the sidewall of a truck tire or casing includes information that restricts the maximum speed, then the maximum is 75 mph.

Heat is the single most important factor when it comes to the performance of truck tires; when speeds are higher, the possibilities of heat-related problems are going to increase. Manufacturers have a built-in safety factor, so short-term periods where the speed may exceed the 75 mph maximum should not be a problem if the inflation pressure is sufficient for the load. Of course, if the inflation pressure cannot support the weight of the vehicle and its contents, then excessive speed is only going to determine when, not if, the failure occurs.

Contrary to recent investigative reports, the sky is not falling as the result of higher speed limits. Highway safety has not been compromised because a few states have limited stretches of road where the speed limit for trucks exceeds 75 mph. That being said, fleets that operate trucks without governors in states with speed limits in the 80-85 mph range must understand the margin of error for overloaded tires will be much smaller. Slightly underinflated tires will generate more heat, which will probably lead to more belt separations at higher speeds.

Durability is the name of the game for truck tires and casings, so speed is not going to be a major factor for most fleets in North America. And when the 75 mph speed limit is exceeded for minimal distances, there is still a built-in safety factor that will prevent most truck tires and retreads from spontaneously coming apart on the highway.  
As long as there is enough air in the tire.

Kevin Rohlwing can be reached at [email protected]

About the Author

Kevin Rohlwing

Kevin Rohlwing is the SVP of training for the Tire Industry Association. He has more than 40 years of experience in the tire industry and has created programs to help train more than 180,000 technicians.

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