The Nissan Leaf electric car, which has been given an official EPA fuel economy rating of 99 mpg, will be introduced in some U.S. markets in December. All cars must be given an EPA fuel economy label rating in order to be sold in the United States.The EPA created the “mpg-equivalent” rating because the Nissan Leaf uses electricity and not gasoline for fuel. The 99 mpg rating will make Leaf the