Japanese auto manufacturer Honda launched the first commercial production of a hydrogen fuel cell-powered car Sunday in Tochigi, Japan, and said its first customers will be in the United States. The four-door sedan, called the FCX Clarity, runs on electricity from a fuel cell battery powered by hydrogen. Steam is its only by-product.    The car boasts a combined (city and highway driving) fuel efficiency of about 72 miles per kg of H2 which, according to Honda's own estimates, is the equivalent of getting about 74 mpg from a gas-powered car.