New York Times: About two months before two new plug-in cars go on sale in the United States, the federal government is struggling with how to rate the fuel economy of mass-market plug-in vehicles. How the Environmental Protection Agency rates the two cars, the Chevrolet Volt and Nissan Leaf, could have a big influence on consumers’ perceptions of vehicles that run on electricity. General Motors, which makes the Volt, and Nissan are anxiously awaiting the agency’s decision as they start production of …