What You Need to Know About Low Rolling Resistance Tires

Tire companies are jumping onto the low rolling resistance (LRR) bandwagon, marketing at least one tire that they claim is more fuel efficient than the rest. But what is low rolling resistance, and how can you compare the fuel efficiency of, say, Bridgestone's Ecopia and Yokohama's Avid Ascend? What about other tire resistance acronyms, such as RRF and RRC?

What Is Rolling Resistance?

Car engines generate energy, much of which is lost somewhere along the line. A great deal of energy is lost in the engine and the power train, but some energy makes it to the tires and is used to move the car. Rolling resistance measures how much of the energy that makes it to the tires is lost to friction with the road surface and to the process known as hysteresis.

Hysteresis refers to how tires flex as weight is placed on them and then snap back into shape as they roll. Because of the laws of physics, the energy that returns to the tire when it snaps back is always less than the energy it took to deform it, so the tire loses energy from flexing at every moment it is moving. As much as 30% of the energy that makes it to the tires is given up to friction or hysteresis.

Ultimately, all energy provided by the engine comes from the gas tank, which is why trying to retain that energy is so important: The more energy that goes to moving the car, the better the car's fuel mileage will be. With gas prices remaining high and environmental considerations increasing in importance, fuel efficiency is the name of the game. It's difficult to further reduce the friction in the engine and powertrain, making the tires one of the best areas to try to regain lost energy.

Originally, low rolling-resistance tires were made with a hard rubber compound and stiff sidewalls to reduce friction and flex. While this approach worked moderately well in reducing friction, using those tires felt like riding on rocks, and they had very little grip. Nowadays, new tire compounds, such as those based on silica and alternative oils. are showing good rolling-resistance properties while maintaining a pleasant ride and better grip.

Other Measurements

Rolling resistance force (RRF) and rolling resistance coefficient (RRC) are two factors often used to evaluate the rolling resistance of tires. RRF is the force in pounds or kilograms required to rotate a tire at 50 mph against a large steel drum, while RRC is obtained by dividing RRF by the load placed on the tire. Using these numbers to compare tires is complex. While RRF is easy to compare, it doesn't take into account the size and load of the tires. RRC takes these factors into account, but it makes it impossible to compare tires of different sizes.

This is why tire companies often market LRR tires using fuzzy comparisons, such as claiming that their tire is “20% more fuel efficient than the competitor's tire” or offers “10% less rolling resistance than the previous tire.” These numbers are generally either an average RRC across an entire line of tires or a best-case scenario for a particular size, making comparisons difficult.

Fuel Efficiency

Present LRR technology will give a fuel-efficiency improvement of 1 to 4 mpg at best. While this might not seem like much, taken over the life of the tires it adds up. There are, however, some important issues to remember.

First, if you read online discussions of LRR tires, you will inevitably see complaints that a consumer's new LRR tires give worse fuel mileage than their old standard tires. Put simply, worn tires have much lower rolling resistance than new tires. When you replace old tires with new ones, your fuel mileage will always drop, regardless of how low the rolling resistance on the new tires is. The only fair comparisons are between brand-new tires or between tires worn to the same degree.

Second, two related factors are as important to fuel efficiency as the tires themselves:

• Motor Oil: Using the correct weight of oil in your car will cut down on engine friction, accounting for at least as much fuel-efficiency gains as the tires.
• Tire Pressure: Even slightly underpressurized tires will quickly bleed away any fuel savings from LRR tires. To get the best fuel savings from LRR tires, it's best to check the pressure every time you fill the tank.

LRR tires appear to be an effective and useful new technology, even if they are in their infancy. With high gas prices, it's good to have tires that save even a bit of fuel while they keep your car rolling smoothly.