The 19-year-old EPA tests for city and highway mileage actually gauge vehicle emissions and use that data to derive an estimated fuel-efficiency rating. The EPA tests pre-production vehicles in a lab to simulate vehicle starts and stops on crowded city streets and open road conditions. According to the EPA website, "The tests measure the waste substances emitted from consuming the fuel, not the actual fuel consumed. From the measurement of emissions, EPA can estimate the miles per gallon achieved by the vehicle on average."
Your mileage may still vary, but at least EPA estimates will soon give you more realistic expectations. For the first time in 28 years, the EPA is updating the testing procedures used to calculate the average miles per gallon estimates that auto makers use to advertise their vehicles.
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http://www.fueleconomy.gov/feg/fe_test_schedules.shtml