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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286
thereby seating and sealing the tire on the rim. Encircling the tire is the tread. This is a thick band of rubber that forms the tire surface, from its crown (its largest radius) to its shoulders (the areas in which the tread transitions to the sidewalls).
The tread is the only part of the tire that comes in contact with the road surface during normal driving. The tread band consists of a grooved section on top of a base. The tread’s design, including its grooved pattern, helps in the removal of road surface water and other contaminants from under the tire while maintaining an adequate level of frictional adhesion between the tire and road to generate torque, cornering, and braking forces under a wide range of operating conditions. For most passenger tires, the grooves start out 9/32 to 13/32 inch deep. Tires are normally considered worn when only 2/32 inch of tread remains.
Most steel-belted radial passenger tires weigh more than 20 pounds, and they can exceed 50 pounds. The steel typically makes up about 15 percent of the total weight, the cord material another 5 percent, and the rubber compound in the carcass and tread about 80 percent (Modern Tire Dealer 2006, 51). Most of the rubber compound’s weight is from natural and synthetic polymers and reinforcing fillers. Other materials added to the compound during processing, such as oils, can contribute 3 to 25 percent of its weight. Because these compounding materials can account for about half of a tire’s total production cost, fluctuations in material prices can have important effects on tire retail prices (Modern Tire Dealer 2006, 46).
The largest application of pneumatic tires is on highway vehicles, which consist of heavy and medium trucks, commercial light trucks, and cars and light trucks used as passenger vehicles. Heavy and medium trucks range from buses to tractor-trailers and construction vehicles. Their tires are designed for heavy workloads, long-distance travel, and rough terrain. Commercial light trucks include many full-size pickups and vans, as well as some SUVs. Their tires are designed mainly for rough terrain and heavy loads. Cars and light-duty trucks used for passenger transportation are the most common vehicles on the highway. Their tires are designed mainly for ride comfort, traction, handling, and wear life, as well as appearance and affordability.
The focus of this study is on tires used on passenger cars and light-duty trucks. The federal government defines and regulates these passen-