If you've been shopping for replacement tires recently, you've probably noticed a lot of emphasis on fuel efficiency and environmental impact. The rubber itself is still black, but tires are becoming significantly greener. Although many drivers are only now becoming aware of low-rolling-resistance tires, their development began in the 1990s, once automakers had gone after some of the fuel economy modifications that were easier to achieve, such as reducing vehicle weight and improving engine efficiency and aerodynamics.
Unfortunately, there is no easy way for consumers to navigate all the "greenery" currently sprouting up in the world of replacement tires. Even worse, many consumers remain unaware of the possible trade-offs involved. If you get tires that are more fuel efficient, are you giving up wet traction performance? It's not always easy to know which green tires are right for your car.
If your vehicle came from the factory with low-rolling-resistance tires, simply insist on original-equipment tires as replacements. These tires typically are available from the dealer, as well as from online sources and local tire retailers. If you're looking to make the switch to low-rolling-resistance tires, the answer isn't so straightforward. They can come with performance trade-offs, and for a proper explanation, a little physics lesson is in order.
Rolling Resistance Defined
Simply put, rolling resistance is the force required to keep your vehicle's tires rolling at a given speed. Tire manufacturers quantify it by rolling a tire against a large cylindrical drum and measuring the forces involved. The result is the tire's rolling resistance coefficient (RRC).
Tires change shape as they rotate, and the portion of the tire in contact with the road is deformed before it returns to its relaxed state. The energy required to deform a tire is greater than what's needed to return it to its original shape: a phenomenon known as "hysteresis." This energy is dissipated in the form of heat, and this heat plays a major role in rolling resistance.
If you've ever pedaled a bicycle with an underinflated tire, you have firsthand experience with hysteresis. To cruise at a constant speed, you had to put more mechanical energy into the system, pedaling harder than if the tire had been inflated to its proper level. That's because underinflated tires have lots of hysteresis, creating more rolling resistance.
So, if underinflated tires have high rolling resistance, then why not simply overinflate them to reduce their rolling resistance? That works, but there's a price to pay. For one thing, ride quality suffers, becoming increasingly harsh as tire pressures rise. More importantly, the higher the pressure, the smaller the "footprint," which is the contact patch between your tires and the road surface. A smaller contact patch can mean less traction, which translates to decreased braking and cornering performance, especially on wet surfaces.
Reducing Resistance
Engineers have several options for squeezing every bit of rolling resistance out of a tire. Reducing a tire's footprint is perhaps the most obvious, but as noted, that can have the undesirable consequence of reduced traction. A tire's composition also plays a factor, and the recent addition of silica to the typical compound of natural and synthetic rubbers has decreased rolling resistance without sacrificing traction.
Reducing the depth of a tire's tread blocks is another fuel-saving tactic. The deeper the tread blocks, the more they tend to flex as they go down the road. Imagine a set of beefy off-road truck tires, and you'll get the idea. Such tires are good for grip but bad for fuel economy, tire life and road noise.
Sidewall design also can help: Stiffer sidewalls tend to reduce rolling resistance, but this also increases mass, which can hurt rolling resistance.
Resistance Wastes Fuel
Much of the energy that actually makes it from your car's gas tank to its wheels is used to overcome inertia (acceleration and deceleration) and aerodynamic drag. Your tires' rolling resistance uses up about 4 percent of that energy around town, rising to approximately 7 percent on the highway. How does this translate to fuel economy and dollars and cents? A long-established rule of thumb holds that a 10 percent decrease in rolling resistance yields a 1-2 percent improvement in fuel economy.
That doesn't sound like much, but it can add up. The average American still drives about 12,000 miles each year, consuming about 550 gallons of fuel in the process, according to the federal Energy Information Agency. If gas were $3.50 per gallon, that 2 percent savings would translate to $19.50 to $38.50 each year. Over the three- to four-year lifespan of a set of tires, this can mean $58.50 to $154 in fuel savings. That's something to think about next time you're in the market for tires.
For automakers, that 1-2 percent improvement is far more valuable. Even a slight bump in fuel economy on a high-volume model can go a long way toward bringing up the manufacturer's Corporate Average Fuel Economy (CAFE), helping the automaker avoid costly government fines. But while CAFE forces automakers to think seriously about the rolling resistance of their OEM tires, replacement tires rarely have received the same scrutiny.
Do Low-Rolling-Resistance Tires Save You Money?
Fuel savings also can overcome any price premium you might pay for low-rolling-resistance tires, which are sometimes called fuel-efficiency tires. When they first hit the market, these tires almost always cost more than similarly sized "conventional" tires: New technologies typically come with a cost premium.
Today it is harder to make a blanket assertion that tires with lower rolling resistance cost more than others. Some do, while others don't. It all depends on the rubber compounds and other materials used to build the tire, the tire's structure, the tire maker's marketing strategy and whether you are looking at the initial price tag or the overall cost of ownership, says Daniel Guiney, director of technical services for Yokohama Tire Corporation.
Low-rolling-resistance tires can have less tread life than comparable conventional tires (although some high-performance tires also have terribly short lives). But even then, their fuel savings can help make up for lesser longevity.
Take a set of four low-rolling-resistance tires that costs $500 and has an expected life of 50,000 miles and compare it with a $400 set of conventional tires with a 60,000-mile tread-wear rating.
The 60,000-mile tires sound like a real bargain. But if each set of tires is mounted on a 25-mpg car that's driven 12,000 miles a year, and the low-rolling-resistance tires deliver a 2 percent annual fuel savings, they will save its owner $33.60 a year in gasoline costs if gas is $3.50 a gallon. The low-rolling-resistance tires will have to be replaced after 50 months, while the conventional tires will last 60 months. The total cost of ownership for low-rolling-resistance tires, however, is 8 percent less than for the conventional tires when the fuel savings are taken into account.
As fuel prices rise, or if the gaps between tread-wear ratings and purchase prices are narrower than in that example, the cost premium of low-rolling-resistance tires quickly disappears.
Federally Required Labels Are Coming — Very Slowly
The range of rolling resistance is considerable for replacement tires, with up to a 25 percent spread between the lowest and highest amounts of resistance. That could translate into a 5 percent difference in fuel economy among otherwise similar tires. Work out the math and the purchase of the tires with the least rolling resistance would potentially save almost $100, assuming that $3.50 per gallon gasoline price.
Unfortunately, consumers don't yet have a way of knowing a tire's rolling resistance measurement. Tire labeling that's required for traction and wear ratings is of little help here. "Hard" tires that last longer do not necessarily have lower rolling resistance, and a higher traction rating doesn't always correspond to higher rolling resistance. What's a consumer to do? Help is on the way, thanks to legislation aimed at better informing consumers about tire issues.
The national effort has been a slow one, however. Congress passed a law requiring some sort of tire-efficiency labeling in 2007 and the National Highway Traffic Safety Administration issued testing procedure rules for tire labeling in 2010. But NHTSA, consumer groups and tire manufacturers have been negotiating ever since then on just what the labels should say, how they should look and how they should be displayed. A final set of rules governing all that is expected sometime in 2014, an agency spokesman said.
It is likely that whatever label is decided, tire makers won't have to imprint it on the sidewall of each tire, as they currently do with the Uniform Tire Quality Grading system. That system includes a numeric rating for tread wear and alphabetic ratings for traction and heat resistance. Instead, proposals include a paper label to be affixed to new tires, or a tire ratings book that retailers would keep on display so customers could look up the tires they were considering and review the new ratings.
A Performance Compromise
Many tire makers welcome rolling resistance labeling on tires, although there is widespread concern that oversimplified labeling would misrepresent the complexities of tire performance. If the labels focus solely on fuel economy and environmental benefits, consumers might overlook other important performance aspects, such as wet braking performance, they say.
Edmunds.com's own testing of vehicles equipped with both regular tires and low-rolling-resistance tires has often revealed performance differences in terms of handling and stopping distances.
The sample labels that NHTSA has been offered for review all include a rating for safety (also described as wet traction), durability (also described as tread wear) and fuel economy (also described as rolling resistance). There's no stopping-distance rating because stopping distances depend on the interplay of many elements, including the road surface and a vehicle's tires, brakes, weight and speed.
How To Shop Now
When it comes to buying replacement tires, the advice regarding low-rolling-resistance tires is the same as for regular tires. For the vast majority of us, the tires that came from the manufacturer are often the way to go, as they have undergone extensive testing on that particular vehicle. Those who simply can't leave well enough alone, or insist on entering their Priuses in weekend autocross events, should be sure to do their tire homework.
Presumably, once a national labeling system is introduced, this task should become easier. For now, you can do research via online communities such as the Edmunds Forums and useful resources such as Tire Rack, which goes beyond the usual sizing information and pricing by providing extensive test results and customer reviews. Being aware of the trade-offs between fuel economy and tire performance will make you a savvier tire shopper.
