Our gasoline-burning cars are the second-largest source of U.S. global warming pollution. But Americans will put more than 300 million new cars on the road over the next 20 years -- if these cars are the best, most efficient vehicles Detroit can make, we'll take a big step toward solving global warming. Using hybrid engines and other ready-to-go technologies in today's cars could nearly double the mileage they'd get from a gallon of gas, saving a lot of money at the pump. By 2050, fuel-cell technologies and other advancements could boost efficiency to 54 miles per gallon. We can curb our appetite for oil even further by adopting "smart growth" principles in our cities and towns, encouraging developers to build compact, walkable communities that allow people to spend less time behind the wheel. It Can Be Done: » Guide to Hybrid Cars » How Smart Growth Solves Sprawl » Move America Beyond Oil Step 3: Biofuels & Renewable Energy.