Tires can use as much as 15% of the total energy produced by the engine. It takes about 10 horsepower to overcome the rolling resistance of the tires at 100 km/hr. It comes as no surprise therefore that tire companies are developing and marketing tires that are “green” in terms of improving fuel mileage and thus reducing exhaust emissions.
During stop-and-go city driving, overcoming inertia makes up 35% of the total resistance to motion with driveline friction (engine and transmission) accounting for 45%, aero drag about 5% and tires (rolling resistance) 15%. At highway speeds overcoming inertia plays an insignificant role and overcoming driveline friction drops to about 15% of the total. But aerodynamic drag jumps to 60% and rolling resistance to 25%.
On a global scale the significant of improving fuel consumption through tires with reduced rolling resistance not only reduces our consumption of fossil fuels it thus reduces the amount of exhaust emissions. Each year more than one billion tires are produced and sold, three-quarters of them for passenger vehicles.
The role played by tires in the drive for improved mileage dates back to the very first pneumatic tire, developed in Ireland in 1888 by a Scotsman named Dunlop. It not only improved the ride of his bicycles, it took less effort to rotate than the solid rubber tires in use at the time. Tires have undergone continual development ever since. Plies of cotton cord were introduced in the first part of the 20th century, radial ply construction replaced bias plies in the 1950s, the inner tube was eliminated, fewer but stronger cords and more dimensionally consistent production have all played incremental roles as has the trend toward larger diameter tires with reduce rolling resistance.
The radial tire brought a 20 to 25 percent improvement in rolling resistance accompanied by a 4-5% reduction in fuel consumption. The gains made with construction, production, materials and design since then have meant a further 8 – 10% cut in rolling resistance and 1% better mileage.
We have reached a stage where science and technology play an ever-increasing role in this game. Today’s tire is made up of about 50 different materials. The tread design and compound provides the traction, the sidewall plays a major role in ride quality and both contribute to rolling resistance. The challenge facing scientists and engineers is the trade-off between increasing friction or grip and reducing rolling resistance. They can arrive at a design, compound or formula that reduces rolling resistance, but normally at some the cost – reduced wet traction and tread life.
Low inflation pressures can negate all the work done in the design and development stages. Fuel efficiency is reduced by 1% for every 3 PSI of under inflation.
Tire pressure will drop one pound per square inch (PSI) for every 10 degree F drop in air temperature. They also loose pressure over time, as much as 1.5 psi per month. The US Energy Department has said that under-inflated tires waste an estimated 4 million gallons of gas daily in America. A study by the AAA, of more than 11,000 vehicles across the US showed more than 40% of passenger cars and 45% of trucks, SUVs and minivans have at least one tire under inflated by 6 psi or more. A vehicle with a recommended pressure of 35 psi whose tires are at 28 psi will have increased its rolling resistance by 12.5%.
Ambient temperature also plays a role in rolling resistance. Tests by Bridgestone indicate that the rolling resistance of a given tire at -20 C is more than twice that at 40 C.
Transport Canada says the use of low rolling resistance (LRR) tires as replacement tires can also help to reduce fuel consumption in vehicles by 4-5%.