In one of the last steps toward selling gasoline with higher concentrations of ethanol, the EPA finalized a label that will identify gas pumps dispensing E15 ethanol, a fuel containing 15 percent ethanol and 85 percent gasoline. (Read: "Move over E85, here comes E15.")

The new orange label displays "E15" in large type and states that the fuel is for use only in 2001 or newer model-year vehicles or flex-fuel vehicles, and that it is illegal to use it in other vehicles or in power equipment such as lawnmowers.

In response to the release of the labels, nine automakers—including Chrysler, General Motors, and Toyota—wasted no time writing letters to Congress criticizing the proposal and noting that they will not honor warranties for older cars running on E15. The automakers say they are concerned about the effects of E15 on engines, fuel pumps, and other fuel-system components in cars that were not designed for it. (Learn more about ethanol: "The great ethanol debate.")

In January, the EPA approved the use of E15 in all cars from the 2001 model year on. The only cars that would be warranted for use of the new fuel are flex-fuel vehicles, which are designed to use concentrations of ethanol up to 85 percent (E85).

Since many of the older cars approved to use E15, however, are out of warranty by now anyway, the main disagreement among automakers and legislators centers on the use of E15 in newer cars still under warranty. However, consumers are concerned whether 2001-and-later cars out of warranty may encounter higher repair costs using E15. Most regular gasoline sold in the United States, especially in the summer, is already blended with up to 10-percent ethanol (called E10), which automakers do allow under warranty coverage.

"I don't know that owners would have a good experience" using E15 in older cars, says Wade Newton, a spokesman for Auto Alliance, a lobbying group for automakers in Washington, D.C.

Even with the EPA approval and the new labels, other hurdles remain for gas station operators to begin pumping E15. Because stations will need to continue to provide regular fuel for older cars and power equipment, they would need to install a new underground storage tank and pump to offer E15—a significant expense.

The common objections to E15 point out how long and difficult the road will be to wean the U.S. economy from imported oil. As a liquid fuel that has been produced for centuries, ethanol should require the fewest changes from traditional petroleum among combustible alternatives.

Ethanol has promise as a renewable, locally sourced fuel. And it doesn't have to be derived from food stock. In fact, companies are working on commercializing ethanol made from sources other than corn in the United States. (In Brazil, Sweden, and South Africa, most ethanol is made from other sources.) But laws, standards, technology, and the legacy automotive fleet would all have to change to accommodate more ethanol in our national fuel portfolio. Even moving to gasoline or biodiesel made from algae or soy beans is likely to run into similar constraints. Clearly, the shift away from a gasoline-dependent national fleet will take time, commitment, and investment.

See our guides to alternative fuels and fuel economy.