Change Car. Mazda is the most fuel-efficient automaker in the U.S., says the EPA. In its latest “Light Duty Fuel Economy Trends” report, the EPA said Mazda was tops when it came to fuel-efficiency, with a fleet-wide adjusted fuel economy average of 27.1 mpg, and CO2 emissions of 328 grams per mile (g/mi) for the 2012 model year.