Bridgestone engineers are currently testing eight-inch-diameter airless tires for low-speed industrial applications such as carts and forklifts and developing a larger test version for passenger cars, but plenty of challenges remain. The engineers have to ensure that the tires are durable in extreme heat and cold. They will have to find a way to keep rocks, mud and snow from collecting in the spokes, which could unbalance the tire. Then the company must convince drivers to accept them. Not surprisingly, Bridgestone won't say when the tires might appear in showrooms. But if they do reach the market, drivers could save fuel, never again change a flat, and start cutting down the heap of 300 million pneumatic tires that are scrapped in the U.S. every year.