NASA did it again - "...which means that an ion thruster has a fuel efficiency of 10-12 times greater than a chemical thruster..."
http://www.nasa.gov/multimedia/imagegallery/image_feature_2416.html#.UhMDiFLqxk9
http://www.nasa.gov/multimedia/imagegallery/image_feature_2416.html#.UhMDiFLqxk9