The challenge: Build the world's most fuel-efficient production car -- one that gets maybe 250 miles per gallon and causes little or no pollution. The payoff: prize money from the group that awarded $10 million for the world's first private spaceflight two years ago. Ford's Model T got 25 miles per gallon, and today a Ford Explorer gets 18 miles per gallon," says Peter Diamandis, X-Prize Foundation chairman. "We believe the time is ripe for a fundamental change in what we drive -- and we believe an X-Prize in this area can drive a substantial change." U.S. agencies procure new technology mostly through contracts with universities and companies. Taxpayers typically pay, whether or not companies or researchers actually succeed. But government interest in prizes began to grow after 1996, when the $10 million Ansari X-Prize was announced for a privately financed craft to fly into space. That means inspiring the likes of Felix Kramer, a California Internet entrepreneur who hopes to partner with a big auto company to create a high-mileage car. That's what his CalCars team did in September 2004, when it developed the prototype of a Toyota Prius with an electric cord. The extra reliance on electric power gives the "Prius-plus" better than 80 miles per gallon.