The Obama administration proposed on Wednesday doubling auto fuel efficiency to 54.5 miles per gallon by 2025, a White House energy priority that has come under scrutiny in Congress.

The plan grew out of an uneasy agreement between the administration, automakers and environmental groups to reduce U.S. dependence on oil imports and cut tailpipe emissions.

Regulators hope to finalize the proposal by summer following a 60-day public comment period. The administration wants to give industry five years to develop fuel-saving technologies further and plan products before the rule would start taking effect in 2017...

Turning the energy of the sun into motion is an idea that long has captured the human imagination. Of course, the sails on boats capture the form of solar energy known as wind. But to make a car move on sunlight requires a wide, flat surface on which photovoltaic (PV) panels can work their magic. As a result, solar cars are as striking in appearance as the energy efficiency they can achieve.

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At the former Humberstone saltpeter works in northern Chile this fall, 11 teams from five countries gathered with an array of solar-powered contraptions on wheels. Their mission was to compete in a historic race of vehicles powered by nothing but two of the oldest energy sources known to man: muscle power and the sun.

The race covered 620 miles (1,060 kilometers) over three days in the Atacama Desert, known as the driest desert in the world...