It used to be easy to know whether a car was a glutton for fuel. Federal standards defined miles per gallon and laid out precisely how the statistic should be measured.
But with the dawning of what enthusiasts hope is an era of electric cars, federal regulators are struggling to come up with a definition of auto efficiency for the next generation of technology. This year, two major auto manufacturers, General Motors and Nissan, will bring to market cars that plug in.
"There's no clear answer for how we should be informing the public about the efficiency of these new cars," said Mike Duoba, a research engineer at Argonne National Laboratory, outside Chicago. "The language we have been speaking -- mpg -- isn't sophisticated enough."
To fill the gap, researchers and Environmental Protection Agency officials have been conducting vehicle tests, researching driver habits and even running focus groups toward informing consumers about which cars are energy hogs.
