A study demonstrated a systematic method to assess the degree of concordance between estimated real-world and the U.S. Environmental Protection Agency (EPA) rated fuel economy for light-duty gasoline vehicles based on real-world emissions measurements. Cold engine start effects are accounted for in the real-world fuel economy and in comparison with EPA ratings. Portable emissions measurement systems were used to measure emissions of carbon dioxide, carbon monoxide, hydrocarbons, and nitrogen oxides of 122 vehicles on a specified test route consisting of various roads. Fuel economy of measured vehicles was estimated considering two cases: EPA standard driving cycles and real-world driving cycles. Average cold start emissions measurements from 32 vehicles were used in sensitivity analysis of fuel economy with respect to cold start. Real-world fuel economy estimates and comparison with rated fuel economy are not highly sensitive to cold start effects. On average, both the city and highway ratings are conservative compared with real-world fuel economy. However, there are proportions of vehicles that have lower real-world fuel economy than rated. The method demonstrated should be applied to additional vehicles and real-world driving cycles. Insights from these comparisons could motivate development of a more accurate rating scheme and identification of advice to consumers to improve their fuel economy.