In today's economy, with gasoline prices rising to record-level highs, many consumers and vehicle manufacturers are looking for ways to save on gas consumption and increase fuel economy of their vehicles. Naturally, the question arises as to the role that tires play in a vehicle's overall fuel economy.
First, it is a good idea to understand the difference between OE (Original Equipment) tires that are installed on a new vehicle versus replacement tires. In the United States, vehicle manufacturers are mandated to comply with certain average fuel economy standards for their entire product line or "fleet" of vehicles. For example, the Corporate Average Fuel Economy (CAFÉ), that is mandated by the government for the current year is 27.5 miles per gallon for cars and 20.7 miles per gallon for light trucks, minivans, vans, and sport utility vehicles. Because the CAFÉ is an average fuel economy, vehicle manufacturers must sell both small and large vehicles that have varying fuel economies in order to meet the average fuel economy amounts for their entire product line. If the manufacturer's fleet sales have a better fuel economy than the mandated average, the vehicle manufacturers can earn "credits" towards future years. Conversely, if the average fuel economy falls below the mandated average, the vehicle manufacturer can be fined.
As a result, manufacturers look for ways to improve the fuel economy of their vehicles. Fuel economy is the result of overcoming restrictive forces on the vehicle. These forces include:
In other words, to achieve the same overall performance, heavier vehicles will require more fuel than lighter vehicles, especially on inclines. Generally speaking, vehicles with a "boxier" design will require more fuel than sleek, aerodynamic vehicles. Stopping and starting in city driving will require more fuel than steady driving on the highway. And finally, the rolling resistance of tires can affect fuel economy.
Because the CAFÉ standards are so important to vehicle manufacturers, many of the manufacturers demand that the tire manufacturers develop tires with low rolling resistance to be used as Original Equipment on new vehicles. Consequently, OE tires may be constructed with slightly thinner sidewalls and shallower tread depths, as well as special materials and tread compounds in order to decrease weight and improve rolling resistance.
The question is, how much does the rolling resistance of tires affect the overall fuel economy? First, understand that the CAFÉ testing is performed in a laboratory under ideal conditions. The real-world fuel economy is affected by the factors described above, as well as the driving conditions.
For example, during city driving with frequent stops and starts, it is estimated that one of the greatest factors affecting fuel economy is overcoming inertia, which is responsible for about 35% of the vehicle's resistance. Driveline friction accounts for about 45% of the vehicle's resistance, whereas air drag is relatively negligible at about 5% due to low speeds. That leaves about 15% for the tire rolling resistance.
Contrast city driving to steady highway driving. In this case, overcoming inertia from starting and stopping is practically negligible. Instead, the greatest resistance force is air drag, which accounts for about 60% of the resistive force on the vehicle. Driveline friction is about 15% and tire rolling resistance is closer to 25%.
To translate rolling resistance into a more tangible value, let's consider the following example. Assume you are purchasing a new set of tires to replace your OE tires. Let's further assume that the new tires are high performance tires that increase rolling resistance by about 18-20% compared to the OE tires. According to our estimates above, the rolling resistance of tires has a different effect on fuel economy in city driving compared to highway driving. In order to calculate the difference, we need to multiply the 18-20% difference in the tires by the amount of influence in the vehicle's overall resistive forces:
City Driving = (15% Rolling Resistance Amount) x (18% to 20% Increase) = 2.7% to 3.0% Highway Driving = (25% Rolling Resistance Amount) x (18% to 20% Increase) = 4.5% to 5.0%
Assume your vehicle normally gets 24 miles/gallon in city driving and 31 miles/gallon for highway driving. With the new set of tires, your fuel economy would be affected as follows:
City: (24 mpg) x (2.7% to 3.0%) = reduction of 0.65 mpg to 0.72 mpg Highway: (31 mpg) x (4.5% to 5.0%) = reduction of 1.40 mpg to 1.55 mpg
Clearly, the effect on city driving is not too great, but highway driving is affected substantially. If you drove 20,000 miles per year and 50% of it was highway driving, and assuming average gasoline costs of $3.75 per gallon, in this example you would use nearly $110 more in fuel per year simply due to the tires. The costs are magnified as fuel prices continue to increase.
It must also be mentioned that one of the best and easiest ways to improve your rolling resistance (or at least minimize it) is to keep your tires properly inflated to the correct tire pressure. Under-inflated tires can experience dramatic increases in rolling resistance. For example, if your vehicle recommends that the tires be inflated to 35 psi and the tires are actually under-inflated at 29 psi, they will experience an increase in rolling resistance of approximately 12%.
In summary, not only is it important to choose tires with good rolling resistance characteristics, it is equally important to keep your tires properly inflated to the vehicle manufacturer's recommended air pressure.