Several consumer groups are threatening to take the Environmental Protection Agency (EPA), the federal bureaucracy responsible for those optimistic fuel economy numbers on the window stickers of all new cars sold in America, to court, claiming that the EPA is falsifying its gas mileage estimates. People who are buying fuel-efficient vehicles and expecting to get the advertised fuel economy are disappointed in the accuracy of those numbers, even when they adhere to guidelines designed to maximize efficiency.

For example, last March, AAA issued its own gas mileage estimates based on in-house mileage testing. According to AAA spokesperson Mantill Williams, "We give them our estimate, our real-world estimate, because our auto reviewers aren’t very confident in that EPA number." In another example, the September issue of “Consumer Reports” magazine published a series of articles on EPA mileage estimates and how they compare to real-world results. It found that 90 percent of all vehicles surveyed got worse than expected mileage, with the biggest discrepancies occurring in city driving, where the data was sometimes up to 50 percent lower than EPA estimates. The AAA and “Consumer Reports” experience jibes with Autobytel’s real-world vehicle testing, too.

In fairness, the EPA has established a consistent, measurable environment in which to determine estimated fuel economy data. A trained person drives every car, truck, and SUV on a dynamometer (a treadmill for a car) inside a building where weather conditions are erased from the equation. Two different tests and gas mileage estimates are performed for each vehicle. There is a city test, designed to represent everyday city driving and there is a highway test, designed to represent driving in a rural setting. The dynamometer adds rolling resistance to the vehicle’s drive wheels to simulate the different driving conditions.

The test for city driving simulates a 7.5-mile stop-and-go trip, with an average speed of 20 mph. The trip takes 23 minutes and has 18 stops. About 18 percent of the simulated driving is spent idling. The highway test represents a mixture of ‘non-city’ driving. Highway and interstate driving simulation is designed for a distance of 10 miles at an average speed of 48 mph. There’s very little idling time and the engine is fully warmed up. After the numbers are crunched, the EPA says that it adjusts the laboratory-controlled results down by 10 percent for city driving and 22 percent for highway driving to better establish a real-world estimate. Also, note that the test data is compiled based upon tailpipe emissions, or actual waste from burned fuel, rather than the amount of fuel consumed.

One of the major reasons that the EPA’s numbers are optimistic is that the testing attempts to estimate real-world fuel economy in a controlled laboratory environment. Such testing cannot measure the effects of weather, individual driving habits, additional passengers and cargo, topography, and road surface types. The list of real-world variables that affect fuel consumption are too numerous to list, and that’s the point. As mentioned previously, the EPA adjusts the results down by 10 percent for city driving and 22 percent for highway driving, but clearly this adjustment is not as accurate as it needs to be. With the recent increase in fuel prices, this issue is a hot topic, and we anticipate changes to the EPA methodology to make the data more accurate.

What you need to know is this: The EPA fuel economy estimates are exactly that – estimates based on testing under optimum conditions. Where, when, and how you drive will have a dramatic effect on your actual results, and that effect is usually negative. Take solace in the fact that the testing is performed in a controlled environment, so the EPA numbers are consistently inaccurate and still relevant for cross-shopping comparison. Just don’t expect to match them when you get your new car home.