Fuel Economy Harmonization Act This bill revises requirements for calculating Corporate Average Fuel Economy (CAFE) credits earned by a passenger automobile manufacturer for exceeding minimal required CAFE standards in a particular model year. Beginning in model year 2016 through model year 2021, a manufacturer may apply any credits earned after model year 2009 to subsequent model years. The bill revises fuel economy credit transferring program requirements allowing a manufacturer of a fleet that exceeds CAFE standards to transfer the earned credits and to apply them within that fleet to a compliance category of automobiles that fails to achieve prescribed CAFE standards. The maximum increases in any compliance category attributable to transferred credits are: 1.0 mile per gallon, for model years 2010-2013; 1.5 miles per gallon, for model years 2014-2016; 2.0 miles per gallon, for model years 2017 and 2018; 4.0 miles per gallon, for model years 2019-2021; 6.0 miles per gallon, for model year 2022 and subsequent model years. The bill requires the Environmental Protection Agency to modify its calculation of average fuel economy for model years 2012 through 2025 at the request of a manufacturer for its passenger and nonpassenger automobiles.