Tire Rolling Resistance Part 1: Understanding Corporate Average Fuel Economy(Lea en español)
In the United States, vehicle manufacturers are required to maintain an average fuel economy for the fleet of new vehicles they sell each year. In 2009, the government Corporate Average Fuel Economy (CAFE) mandate was 27.5 miles per gallon (mpg) for cars and 23.1 mpg for light trucks (including minivans, vans and most pickup trucks and sport utility vehicles). However because it's an average fuel economy, in order to sell large cars or trucks (that use more fuel), the vehicle manufacturer must also sell small cars and trucks (that are fuel-efficient). The vehicle manufacturer can be fined if their annual vehicle fleet uses too much fuel.
A tire's rolling resistance affects fuel economy. Most vehicle manufacturers demand their suppliers develop low rolling resistance tires to be used as Original Equipment on their new vehicles to help average out their CAFE-mandated mpg. In order to meet the manufacturer's demands, these tires are often designed with a priority on reducing weight and rolling resistance and are molded with slightly thinner sidewalls, shallower tread depths and use low rolling resistance constructions and tread compounds.
However, in order to understand CAFE tests and the role that tires play, it is important to recognize that CAFE tests are conducted in a laboratory and not on the highway. Many aspects that affect fuel economy in the real world are reduced to constants incorporated into the formulas specified.
A vehicle's fuel economy is the direct result of its total resistance to movement. This includes overcoming inertia (Newton's Law), driveline friction, road grades, tire rolling resistance and air drag. In order to offer the same level of performance, heavy vehicles require more power (and more fuel) than light vehicles. All-wheel and four-wheel drive vehicles require more power than two-wheel drive vehicles and boxy vehicles require more power than low drag aerodynamic vehicles.
But how much influence does each of these elements have and when are their influences felt? Once you eliminate the fuel converted into heat by the engine, the relative percent of influence that these other factors represent during stop-and-go city driving are very different than during steady-speed, highway driving.
During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%, air drag is about 5% and tire rolling resistance is about 15%.
Overcoming inertia no longer plays an appreciable role in the vehicle's resistance during steady speed highway driving. For those conditions it is estimated that driveline friction is about 15%; air drag is about 60% and tire rolling resistance represent about 25%.
Let's explore a scenario where a high performance replacement radial tire has a whopping 20% increase in rolling resistance over a low rolling resistance Original Equipment standard passenger radial. To calculate the potential change in mpg resulting from using the high performance tires in place of the Original Equipment tires, we would multiply the tire's percentage of influence on the vehicle's overall resistance (15% in the city and 25% on the highway) times the high performance tires' 20% increase in rolling resistance.
If the vehicle equipped with standard Original Equipment, low rolling resistance passenger tires normally provided 25 mpg in the city and 30 mpg on the highway, installing tires with 20% greater rolling resistance would only drop fuel mileage by a calculated 3% (to 24.25 mpg) in the city and a calculated 5% (to 28.5 mpg) on the highway. While this is a measurable difference, it probably isn't much more of an influence on real world fuel economy than being stuck in rush hour traffic a couple of times a week or being stopped at every red light instead of continuing through a string of green lights.
The easiest way to reduce rolling resistance and enhance fuel economy is to make certain that the tires are properly inflated. A vehicle that requires its tires to be inflated to 35 psi (based on the vehicle's tire placard) will have an increase in rolling resistance of approximately 12.5% if the tires are allowed to become under-inflated to just 28 psi. Therefore, maintaining the vehicle manufacturer's pressure recommended for light load and heavy load conditions may almost be as important as the tires being used.