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U.S. government fuel economy tests are used for two primary purposes: 1) to monitor automobile manufacturers’ compliance with fuel economy and greenhouse gas emissions standards and 2) to inform consumers about the fuel economy of passenger cars and light trucks. This study analyzes a unique database of 75,000 fuel economy estimates self-reported by customers of the U.S. government website www.fueleconomy.gov to evaluate the effectiveness of the government's estimates for these two purposes. The analysis shows great variability in individuals’ own fuel economy estimates relative to the official government estimates with a small bias relative to the sample average. For consumers, the primary limitation of government fuel economy estimates is imprecision for a given individual rather than bias relative to the average individual. The analysis also examines correlations between individuals’ fuel economy estimates and specific technologies, vehicle class, driving style, method used to calculate fuel economy, manufacturer, and state. Gasoline, hybrid and diesel vehicles were separately evaluated. There is some evidence that the shortfall between test cycle fuel economy estimates (used to measure compliance with regulations) and in-use fuel economy estimates (such as those provided by customers of www.fueleconomy.gov ) has been increasing since 2005. If this trend continues, it could affect the benefits realized by fuel economy and greenhouse gas emissions standards. A scientifically designed survey of in-use fuel economy is needed to insure that an unbiased sample is collected and that fuel economy is rigorously and consistently measured for all vehicles. The potential for information technology to enable more precise prediction of individual fuel economy should be explored. [ABSTRACT FROM AUTHOR]