Basically, the government now lets the states set their own speed limits. And, in most cases, that means increasing them.
    For example, prior to the repeal of the law, 40 states posted 65 mph as their truck limits, 2 states at 60 mph and 8 states at 55 mph.
     Today-and this includes legislation that's been passed but not yet enacted?10 states have increased to 75 mph and 5 to 70 mph, while the other states have yet to change.
     Speed limits can affect tires. Designing a tire to optimize performance at both 55 and 75 mph becomes a great challenge.

Will Tire And Rim Association Guidelines Help?

 Perhaps, but not likely.
     Current Tire and Rim Association tables are based on a maximum speed of 65 mph.
     But, the Tire and Rim Association has responded to the changing speed limit issue with a proposal (due for a vote this fall) that involves:
     a. Providing load and inflation information at 65 mph only.
     b. Allowing each tire manufacturer to provide a rating for each specific tire rated over 65 mph.
     We view that change as a good thing. We want our customers to have confidence that the truck tires they have selected can meet the requirements of their fleets.

 As reported in the July issue of Tire Business, "the motivation...is increased productivity and driver retention. If a truck can haul freight faster, it follows that it can haul more loads. And past experience indicates that, if all else remains the same, drivers will switch companies if their trucks will go as little as two mph faster."

What Must Be Sacrificed In Order To Go Faster?

A number of things. Some obvious, some not so obvious.
     Fuel economy, for example, suffers. In fact, fuel economy will decrease an average of 0.1 mile per gallon for every 1 mph increase in speed. And, if you multiply that out over the total annual mileage at higher speeds, the cost becomes significant.
     Higher road speeds also mean higher engine speeds, which create additional maintenance requirements. In fact, Tire Business suggests that "maintenance costs may increase as much as 10-15 percent while engine durability may drop 10-15 percent."
     Braking, too, is affected. It takes longer to stop at higher speeds. Brake temperatures can rise decreasing braking efficiencies, and linings may wear faster.
     The tire can also be affected by faster speeds.

Tire and Rim Association guidelines state that if you increase speed, you also need to reduce your tire load rating by 4 percent for speeds between 65 and 70 mph and over 70 mph, by 12 percent. As well as add 5 psi air pressure for speeds over 65 mph.

     What that means is of the four most popular highway tire sizes: 11R22.5, 11R24.5, 295/75R22.5 and 285/75R24.5, only the 11R24.5 load range H tire meets current TRA guidelines for loads of 12,000 pounds on a steer axle. And none are rated to carry 13,000 or 14,000 pounds on a steer axle at 75 mph.

What Are Some Of The General Effects Of Increased Speed On Tire Performance?

Simply, as speed increases tires get hotter and, as a result, tire durability and fuel economy will be reduced while tire wear will be increased.
     This can also increase irregular wear and maintenance costs. And the "heat history" of the tire will increase.

Just What Is "Heat History?"

It's a term that's used to describe the collective amount of heat a tire experiences throughout its life. Which directly affects the durability of a tire during its original life and the integrity of the casing during subsequent retreads.
     Tires remember heat. It gradually ages the rubber in the tire?just like the aging process affects our bodies.
     And this aging cannot be reversed--each new heat input moves the tire closer and closer to the point where it will no longer be suitable for service.
     Running a tire at higher speeds will accelerate this aging process.

What Happens To Casing Durability At Higher Speeds?

 First of all, the "heat history" effect we just discussed is important to a casing's useful life.
     That means the hotter you run your tires, the shorter the casing life you're going to have.

If you run a tire at 55 mph and then run the same tire at 75 mph, you can reduce the tire's removal mileage from 10-30 percent.
     Not only that, but higher speeds increase the potential for irregular wear. The faster you run a tire, the more its footprint can change. In this case, the pressure on the shoulders of the tire is reduced and the load is concentrated more in the center.

     Because the middle of the tread area is more firmly in contact with the road, the shoulders tend to scrub more and irregular wear can result.
      And, the tread also will wear out faster.
     At this point, our testing on tire wear shows that higher speed (higher temperatures) increases the amount of tire wear. How much depends on your operating conditions.

Right, increased speed increases the chance a tire will be damaged if it hits something, such as a rock or pothole.

Do You Have Any Real World Information On Rolling Resistance And Tire Fuel Efficiency At Higher Speeds?

If you have taken steps to improve fuel economy by using shallower tread tires, you may have seen an increase in fuel economy. But when you increase from 55 mph to 75 mph, you give away about half of that gain.
     And that doesn't take into account fuel economy losses resulting from vehicles operating inefficiently or increased aerodynamic drag.
     If you paid more for a tire with lower rolling resistance, high speeds will greatly diminish its fuel economy advantage.

OK, So What Does This Really Mean?

 Bridgestone's recommendation is that before you allow your drivers to increase their speeds, you should carefully consider all of the things discussed in this article. You may also want to consult with your tire supplier.

      We'd like to hear from you about your experiences with tires driven at higher speeds. Or your own feeling about speed controls on your vehicles and drivers. Or your concerns about higher speed gains and losses.
     Please take a few minutes to fill out the questionnaire bound into this issue, and mail it back to us.

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