Biodiesel [B99 or B100]Made from: Vegetable oils or animal fat. Most common sources are soy or canola oils as well as used restaurant grease.
Use: In any diesel vehicle, but use with caution in vehicles built before 1996 because biodiesel can damage rubber seals.
California: Through more than 50 retail stations and other sites, but customers must become members of a co-op or user group. Hours, restrictions and available blends vary.
Pros: Nontoxic; biodegradable; ample domestic material for production; 78% less carbon dioxide emissions than petroleum diesel; cuts petroleum consumption.
Cons: More expensive; quality can be uneven, depending on the producer's process and source material; pure biodiesel and high blends can thicken in cold weather; large-scale use of soy or palm oils could raise food supply and land use concerns; some engine makers limit or won't back warranties on vehicles using higher diesel blends.
Ethanol [E85 blend]Made from: Corn or sugar crops, mostly. But producers are moving toward deriving it from agricultural waste, prairie grasses and other sources. E85 contains 85% ethanol and 15% regular gasoline.
Use: In so-called flexible-fuel vehicles built to run on E85, gasoline, or any blend of the two. There are more than 6 million of them already on the road, such as recent Lincoln Town Cars, Dodge Ram pickups, Chevrolet Tahoes and Suburbans.
Public availability in California: At three gas stations, including Pearson Fuels in San Diego, Conserv Fuel in Brentwood and Stanley's Food Mart in Tulare. Four other facilities serve government fleets and are not open to the public.
Pros: E85 boosts octane, which reduces knocking; cuts overall greenhouse gas emissions compared with conventional gasoline; sharply reduces petroleum use; helps create fueling infrastructure needed for next-generation biofuels; ethanol production yields byproducts such as dried distillers' grains used as animal feed.
Cons: Ethanol contains about 30% less energy than gasoline, yielding lower gas mileage; the petroleum products used to produce and transport ethanol reduce the net energy gains; large-scale use of corn-based ethanol could stress supplies for food and raise land use concerns.
Sources: Department of Energy, California Energy Commission, fuel studies; National Ethanol Vehicle Coalition, Argonne National Laboratory study