With each new model introduction, automakers take pains to point out how much they've done to reduce their vehicles' negative effects on the environment. A rundown of the advances can involve almost every aspect of the car's design and production: carbon dioxide output has been cut; paint application has changed to a water-based process; recycling and waste reduction at the factory has been improved to a point where nothing goes to a landfill.
But when it comes to tires, there has been far less to talk about.
Efforts to burnish the eco-friendliness of tires are growing, though, with an emphasis on reducing the use of raw materials -- particularly the five to 10 gallons of petroleum ingredients needed to manufacture a tire.
When the auto industry sprang to life early in the 20th century, tires were made from natural rubber derived from a renewable source: the latex sap of rubber trees. Over the years, the demand for tires that delivered better performance at a lower price inspired makers to add artificial ingredients -- synthetic rubber and special additives that chemists cooked up from crude oil -- to the mix.
A century later, the research laboratories at several tire makers are making progress toward reversing the trend of increased petroleum content. One of the first was Sumitomo Rubber Industries, which has a global alliance with Goodyear, with the Enasave tire it introduced in Japan in 2006. Sumitomo engineers substantially reduced the amount of petrochemicals by cutting the amount of synthetic rubber in half, to 11 percent of the tire's composition from about 22 percent.
Engineers at Yokohama Tire, working toward a similar goal, have come up with tires that are 80 percent petroleum-free, says the company's director of technical services, Dan Guiney. The Yokohama dB Super E-spec, introduced in Japan in 2007, uses chemically modified natural rubber and a processing oil that is derived from orange peels.
In steady driving conditions, the orange oil compound works to reduce rolling resistance by about a fifth over conventional treads, Guiney says, but in cornering and braking maneuvers it quickly generates heat that softens the rubber tread to deliver better grip. The orange-oil-infused tire went on sale in the U.S. last year, and the same technology is used in Yokohama's Advan ENV-R1 racing slick, the specified tire for sports-car road-racing series.
- Reducing resistance: Special rubber formulations and proper inflation have led to improvements in fuel economy.
- New equipment at plants: Tires can be fabricated with much greater precision, which saves weight and thus fuel.
- Recycling: A greater fraction of the estimated 300 million tires that domestic drivers discard each year are being reprocessed and reused.
A non-petroleum oil -- this one more commonly found in the kitchen -- has also become an ingredient in Michelin's tires. The Primacy MXM4 all-weather tire, supplied as original equipment on several Mercedes-Benz and Infiniti models, uses sunflower oil in its formulation for improved traction at low temperatures and shorter braking distances in wet weather.
Elsewhere, researchers at Oregon State University have been working with microcrystalline cellulose, an organic substance that can be made from various plant fibers. It could partly replace silica as a reinforcing filler.
Silica filler is a costly, sophisticated material that requires a lot of energy to produce. The Oregon team successfully replaced as much as 12 percent of the silica used in low-rolling-resistance tires without degrading performance.