The federal government is rethinking the way to calculate fuel-efficiency ratings for electric vehicles, a move that will likely result in a significant reduction in the miles-per-gallon claims some auto makers have touted for the battery-powered cars they plan to launch later this year. Using a preliminary formula released by the Environmental Protection Agency, General Motors Co. last year announced its Chevy Volt would be rated at 230 miles per gallon in city driving, and Nissan Motor Co. said its Leaf electric car would get the equivalent of 367 mpg. But now the agency is in the midst of finalizing...