Tires have a large impact on a vehicle’s fuel economy. Discounting engine and other losses, tire rolling resistance accounts for about 20 percent of the energy needed to power a car and in a country burning the equivalent of 54,093 thousand metric tons of oil a year for transportation that’s a lot of fuel being used to turn tires.
Rolling resistance varies with tire wear, condition, alignment, load, speed, inflation pressure and road texture, so exact calculations are difficult to come by. However, testing proves that low rolling resistance tires not only pay for themselves, but also save money where the rubber meets the road.
The energy losses associated with driving are staggering! According to the National Academy of Sciences, 80 to 88 percent of the energy in your car’s fuel tank is wasted in various thermal, frictional and standby losses in the engine and exhaust systems. This leaves 12 to 20 percent of the potential energy for conversion to vehicle motion. One of the most effective ways to improve this remaining energy’s efficient use is to reduce the rolling resistance of your car’s tires.
Rolling resistance is not a measure of a tire’s traction or grip on the road surface, but rather an indicator of how easily a tire moves down the road, minimizing the energy wasted as heat between the tire and the road, within the tire sidewall and between the tire and the wheel rim.
While the basic rule is “harder tires = lower rolling resistance,” there’s a lot more to the equation than that. You can make your tires “harder” by over inflating them, but that’s not a safe practice. If the tire is too hard or too round, then the tire-patch (which is the area of tire that is in contact with the ground) will be too small to grip the road surface effectively. This in turn will allow the wheel to skid easier when trying to stop or allow the wheel to slip and spin when trying to start from a traffic light.
A tire’s tread design also affects rolling resistance and, more importantly, grip. The more intricate the tread design, the more likely it will grip the road tenaciously. In doing so, the tread traps air in each of the pockets it generates when touching the road surface. When trapping the air/water/dirt/snow in these pockets, the tire is compressing the material that is in the pocket. Therefore, the engine has to do more work to compress these pockets. This makes the tire less efficient to turn and thereby reduces the mileage of the vehicle.
Tire companies are hard at work balancing the rubber compounds and tread patterns they use to find the most fuel-efficient combinations. Along the way, they are discovering new rubber compounds and more efficient tread patterns. Every tire company is working overtime to improve the energy efficiency of their products. Today, there isn’t really a happy medium. If you are an aggressive driver, you are going to be happier with softer tires that “stick” to the road. If you are committed to squeezing every last mile out of a gallon of gas, the harder, low rolling resistance tire is your choice.
If you change your vehicle’s tires, by increasing or decreasing the resistance 20 percent for example, you may raise or lower mileage by three to five percent. While this is a measurable difference, it won’t make as big a difference as making certain that your tires are properly inflated. A vehicle that requires its tires to be inflated to 35 psi will have an increase in rolling resistance of 12 percent or more if the tires are allowed to become under inflated to just 28 psi. Bear in mind that temperature makes a difference. The rule of thumb is for every 10 degrees Fahrenheit change in air temperature, your tire's inflation pressure will change by about one psi (up with higher temperatures and down with lower). When it gets cold, check your pressure to make sure your tire pressure is not too low.
Presently, the biggest problem in shopping for low rolling resistance tires is identifying them! You won’t find any indication of this trait on the sidewall where you can note just about everything else of importance about the tire. The state of California is working on developing a rating for tire rolling resistance, which it plans to require tire makers to print on the tire sidewalls starting next year but how can you purchase low rolling resistance tires right now?
Ask us! As your factory authorized service provider, we maintain information on the OEM tires sold with your car. We also stock “aftermarket” tires including the energy saving, low rolling resistance type. We look forward to consulting with you on the best tires for your car.
