Electric cars are clean cars without any tailpipe emissions. Yes, but there’s one caveat: Unless they’re charged entirely on renewable energy, there are emissions associated with generating the electricity used to power their batteries. This map shows what fuel efficiency would be necessary for a gasoline car to equal the carbon footprint of an electric car on different U.S. regional electric grids. For instance: To emit as little carbon per mile as an electric car charged on the New York state grid, a gasoline car would have to be rated at 160 miles per gallon. That’s impossible. According to the map, only 3 percent of U.S. residents live in a place (mostly in the Rocky Mountains) where a gas car is better for carbon emissions than an electric car.