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May 2011 Pit Stop

Posted in Features on May 1, 2011
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A friend of mine was recently bragging to me about the deal he got on a new set of low-rolling resistance (LRR) tires for his Subaru Outback. As he put it, the fuel mileage he lost by having a full-time all-wheel-drive SUV was cut in half, thanks to the new tread's lack of rolling resistance. I asked him if he noticed any lack of performance. "What do you mean?" he replied. I asked more specifically, "Do the new tires grip the asphalt as well as the old ones?" He said, "Well, of course they do, they're brand new." I could see I was not getting anywhere. So, I changed the subject and concluded that anybody driving an AWD wagon probably didn't know or need to know the finer details of tire performance.

However, this drove me to do a little online research. I wanted to know exactly what makes a tire have low rolling resistance. In doing so, I discovered that LRR tires are as complex as the vehicles they are fitted to. Tread design, carcass construction and compound makeup all have an effect on the effort it takes to make them roll and stop.

A 2003 California Energy Commission study estimated that adoption of low-rolling resistance tires could save anywhere between 1.5 to 4.5 percent of all gasoline consumed in the U.S. each year. However, what this study failed to address is the ill effects that result from using some LRR tires in wet conditions. Rolling resistance is fundamentally the parasitic energy a tire consumes while rolling under load. The phenomenon is complex, and affected by many variables such as ambient temperature, humidity and road surface type. The vast majority of all testing performed on this topic involved dry asphalt. The U.S. Department of Energy estimated that five to 15 percent of light-duty fuel consumption is used to overcome rolling resistance for passenger cars. For heavy trucks, this quantity can be as high as 15 to 30 percent. So yes, LRR tires are a good idea.

However, just as I suspected, in most cases any real-world mpg savings comes at a significant sacrifice to traction in poor driving conditions. Let's not lose sight of what is important here. The tires on your vehicle are the only interface you have with the ground, and surrendering said ability to grip that surface in wet conditions for mpg seems asinine from my perspective.

In an emergency braking scenario, every second counts because every second results in added feet of stopping distance. Why don't we consumers have a system that correlates low rolling resistance to wet stopping distance? When did 3 to 6 percent of added mpg become more important than the extra 30 feet it takes a vehicle to stop during an emergency?

Just such a system has been proposed and is currently in development with the National Highway Traffic Safety Administration. It is known as NHTSA Rule 49 CFR Part 575. Essentially, this ruling would require all tire manufacturers to adopt new test procedures designed to generate data to inform consumers about the effect their tire choices have on fuel efficiency, wet weather safety, and durability.

I believe it is beneficial to all of us if we do something to motivate the would-be tire buyer to perform a little research before heading out to secure a deal on a set of new tires-especially those with low rolling resistance. No tire is perfect, and some consumers are still going to make bad choices regardless of what the label says. However, when my child is crossing through a wet intersection, I want to know that the person behind the wheel of an approaching car has every opportunity possible to stop his vehicle, even if it did cost him an additional two percent of fuel economy. -Robin Stover

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