Natural gas, or simply "gas" in industry parlance, has long been oil's biggest potential rival as a transport fuel. Gas is cleaner than oil—it emits fewer particulates and a quarter less carbon for the same amount of energy output—yet today it powers less than 3 percent of the U.S. transportation fleet (mainly in the form of compressed natural gas, or CNG). This proportion is poised to grow, though, in part because the overall supply of gas keeps growing.
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