The plug-in Focus costs more than the $16,500 base price for the gasoline-powered version and will go on sale at dealers "in the coming few weeks," Eddie Fernandez, a spokesman for Ford, said in an e-mail.

The National Highway Traffic Safety Administration rates the fuel economy of the Focus SFE, a gasoline-powered model, at 33 mpg for city and highway driving. The Environmental Protection Agency certified the electric Focus as offering the equivalent of 105 mpg.

Consumers need to make the decision about whether the higher upfront cost is worth the fuel savings, said Don Anair, the study's author, on a conference call with reporters.

"The cost of the electric vehicles today vary pretty widely based on the models that are out there," he said. "It's important for consumers to understand what the potential savings are on fuel costs, and that can help them make a decision about buying a vehicle."

A high-end Focus sells for about $27,000, or about $5,500 less than a plug-in version after the tax credit, Fernandez said.

Edmunds.com, an auto-researcher, predicts electric vehicles may reach only 7 percent of U.S. auto sales by 2017 even when consumers take rising gas prices into account.

The average U.S. gas price was $3.91 as of Sunday, according to U.S. motorist group AAA. That's a 19 percent increase so far this year.

Passenger vehicles sold in the United States have been getting more fuel-efficient, with the average miles-per-gallon rating rising to 24.5 last month compared with 21.4 four years earlier, according to data compiled by Edmunds.com. Small cars as a percent of U.S. sales rose 3 percentage points to 23 percent, according to the data.

During that time, the average transaction price for an advanced-drive vehicle has risen 6.5 percent to $29,493 from $27,693, according to the Santa Monica, Calif.-based researcher.