Taco grease, please

Ecologists toured Mexico City taco stands and sushi bars on Wednesday to refuel an old school bus with waste cooking oil that will power the next leg of a green-awareness tour from California to Costa Rica.

The bus, which ran on avocado oil during a week-long drive down from the U.S. border, is being used to prove that vehicles can run on recycled fuels that pollute less than gasoline as it chugs around oil refineries, factories and eateries collecting vegetable oil.

Crops like soybeans can also be grown for biofuels -- seen as a transitional energy source that Zaidman said could cover 10 percent of fuel needs, including for things like industry and tractors, for 30 to 40 years. "What with pollution and wars over oil, a lot of people want to get off petroleum," he said.

To convert a diesel engine to run on food oil costs around $400, a much cheaper option than fuel cell cars on the market.