If California wants to achieve its goal of cutting greenhouse-gas emissions 25 percent by 2020 to fight global warming, it must find a way to cut down the biggest source of those pollutants: cars, trucks and other motor vehicles. Even though transportation contributes 41 percent of greenhouse gases, our society is too geographically dispersed and mobile to realistically expect that we can dramatically reduce the number of miles we drive, ride and fly. So that means our vehicles need to become far more fuel-efficient. The less gasoline per mile that a car uses, the less heat-trapping carbon dioxide it spews...