The most fuel efficient car in America is the Mitsubishi i, also known as the i-MiEV.  It's rated at 126 mpg city and 99 mpg highway.  It's an electic vehicle like the Nissan Leaf and the Chevy Volt.  Unlike the Chevy Volt which uses a gas engine in conjunction with the electric motor to get more range, the Mitsubishi i is all electric like it's counterpart the Nissan Leaf.  The Mitsubishi i has a range of around 68 miles per charge.  I have read reviews and people warn against venturing too far out without calculating your trip to minimize the chances of running out of juice.