Automobile Fuel Efficiency Improvements Act of 2003

Automobile Fuel Efficiency Improvements Act of 2003 - Amends the Federal transportation code to phase in an increase in average fuel economy standards: (1) from 25 miles per gallon for passenger automobiles manufactured between model years 1984 and 2006 to 40 miles per gallon for those manufactured after model year 2014; and (2) from 17 miles per gallon for non-passenger automobiles manufactured between model years 1984 and 2006 to 27.5 miles per gallon for those manufactured after model year 2014. Requires the Secretary of Transportation to prescribe such standards for non-passenger automobiles at least 18 months before the beginning of each model year after model year 2015.

Revises the definitions of automobile and passenger automobile, increasing the weight standards and seating capacity.

Increases the civil penalties for violations of fuel economy standards.

Requires the Secretary of Transportation to: (1) report biennially to Congress on the quality of the automobile fuel economy testing for all currently available automobile technologies; and (2) provide for the John A. Volpe National Transportation Systems Center to study what practicable automobile fuel economy testing process provides the most accurate measures of actual automobile fuel economy in highway use, in urban use, and in combined highway and in urban use.

Directs the Secretary to prescribe: (1) a revised testing procedure for accurately measuring the actual automobile fuel economy of each automobile model, based on the latter study; and (2) amended average fuel economy standards taking into account improved accuracy in their calculation resulting from use of such procedure.

Amends the Federal transportation code to revise requirements for the fleet average fuel economy standards for U.S. executive agency automobiles, both passenger and non-passenger.

Directs the President to prescribe regulations that require a specified minimum number of exceptionally fuel-efficient vehicles leased or bought by U.S. executive agencies.