Two recently announced electric cars from General Motors and Nissan promise big advances in fuel efficiency. But the new regulatory picture and some complicated math mean big caveats.

One question is whether the automakers' eye-popping figures will, in fact, end up emblazoned in big black numbers on window stickers. GM is touting 230 mpg for its plug-in hybrid Volt, while Nissan claims its all-electric Leaf will get 367 mpg. If the traditional calculations for city and highway driving still greet car-buying Americans, those stickers could come with a lot of footnotes.

The Environmental Protection Agency, aided by a variety of public and private groups including the National Highway Traffic Safety Administration, will soon release new standards that have been circulating in draft form since February. GM says it used those standards to formulate the Volt's figure, while Nissan says it used NHTSA's current Corporate Average Fuel Economy standards to arrive at its total. A spokesman for Nissan said that had it used EPA's current fuel economy standard (which is used to configure the sticker mileage), the Leaf would have a mileage more around 110 mpg.

But automakers could be required to explain how much gas cars consume in hybrid mode versus "all electric" mode. And, for the first time ever, the EPA is tasked with setting federal standards for greenhouse gas emissions in a "grams per mile of carbon dioxide" measurement so consumers will know how their new ride affects the environment.

The NHTSA, meanwhile, is mandated by law to monitor and enforce a 35.5 mpg CAFE standard by 2016; this measurement will be in the traditional miles per gallon. Thus, electric vehicles' default "kilowatt hour per mile" measurement will be converted to both miles per gallon and grams of carbon dioxide per mile.

Why use a "miles per gallon" metric for a car that doesn't use gasoline? "Consumers understand miles per gallon," said Charles Territo, senior director of communications for the Alliance of Automobile Manufacturers. "Being able to equate benefits in terms of miles per gallon will ensure that consumers understand the benefits of the fuel economy benefits of these vehicles."

Another complication for the new cars is emissions. Under current CAFE standards, only pollutants coming directly out of a vehicle's tailpipe -- known as "downstream" pollutants -- are factored in. That would hardly apply to electric cars. But some environmental advocates want to include other parts of the process. Roland Hwang, transportation program director for the Natural Resources Defense Council, said electric vehicles get 100 percent of their greenhouse gas emissions from "upstream" sources (i.e., power plants that generate the electricity that powers the batteries), while gas-powered cars get roughly 20 percent from upstream sources (the energy it takes to extract, transport and refine crude oil into gas).

"In terms of electric vehicles, you can't ignore the power plant, the electricity generation," Hwang said. "When it comes to gas -- refinery emissions, biofuels -- you can't ignore emissions." Hwang helped craft California's greenhouse gas emissions standard program, which has provided a basis for the forthcoming federal program.

Using California's methodology for scoring greenhouse gas emissions (which incorporates both upstream and downstream sources), Hwang estimates that Nissan's Leaf would plummet to 75 mpg. That might be part of the reason why the EPA concedes that as technology evolves, it will probably need to change stickers again.

That evolution will take time, though. As Territo points out, "There are no plug-in hybrid vehicles being sold in the United States in large volumes." Indeed, experts estimate that there are only 2,000 on the road today; mass-production of the Volt isn't expected to begin until late 2010, and 2012 for the Leaf. Hwang predicts that battery electrics and plug-in hybrids will become a big part of how automakers comply with standards beyond 2016, the endpoint of the forthcoming standards. His speculation also comports with President Obama's goal of having 1 million plug-in hybrid vehicles on the road by 2015. "We have to create the incentives for companies to develop the next generation of clean-energy vehicles -- and for Americans to drive them, particularly as the U.S. auto industry moves forward on a historic restructuring that can position it for a more prosperous future," Obama said in an Earth Day speech in Iowa.

The energy bill sponsored by Reps. Henry Waxman, D-Calif., and Edward Markey, D-Mass., that passed the House in June is a prime example of that development stimulus the president spoke of, according to Felix Kramer, founder of the California Cars Initiative, a nonprofit group that promotes electric cars. "There are incentives and measures that encourage electrification of the transportation industry," he said. "That is really going to accelerate the whole thing." In addition, bills that allocate funding and research programs for advanced vehicle technologies are circulating in Congress, including two separate ones sponsored by Rep. Jim Sensenbrenner, R-Wis., and Rep. Gary Peters, D-Mich. Sensenbrenner's bill focusing on hybrid trucks passed the House unanimously last week.

Rob Peterson, a spokesman for GM in its electric vehicles department, said marketplace competition -- namely between the Volt and the Leaf -- will also help move things along. "Having competition in this space between two major automakers will do two things," Peterson said. "No. 1, it will accelerate the development of the enabling technologies and, two, the supply base required to make sure those technologies will come forward will also increase."

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