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author photo by Nadonnia Jones October 2006

A claim of high gas mileage can attract many a buyer, especially in this uncertain time with unstable gas prices. But before you rush to buy that new hybrid or flexible-fuel vehicle, let’s take a closer look at all the interest in fuel economy.

As mandated by federal law, the miles per gallon (mpg) estimates are clearly displayed for consumers to see when car shopping. “48 city/52 highway” or “19 city/23 highway” that sticker on the window proudly boasts. How is that figure reached? In a lab.

Yes, in a lab. Under controlled conditions, with a standardized testing procedure, auto manufacturers assess their cars’ mileage. The process is simple: a car simulates driving on a machine called a dynamometer, while a hose connected to the tailpipe collects engine exhaust to determine how much carbon it contains. The amount of carbon reveals how much fuel was used during the test, more precisely than a fuel gauge can. The findings are sent to the Environmental Protection Agency (EPA) which then confirms little more than a tenth of the results.

The problem with the dynamometer test is that it does not take into account several factors that affect mpg, including temperature, tire pressure, and going up and down hills. But perhaps the most glaring shortcoming is the fact that the top speed reached on the dynamometer is 60 miles per hour; without question, cars will be driven much faster than that by their owners (which means wind resistance will also be slowing the vehicle).

Those “controlled conditions” are producing numbers far different from what consumers experience in the real world, in bumper-to-bumper traffic with the air conditioner or heater running. Almost 30 years ago, according to fueleconomy.gov, an EPA study showed cars on the road were getting far fewer miles per gallon than lab tests were pulling off. As a result, the EPA required a decrease of 10 percent for city and 22 percent for highway for all new-vehicle mileage estimates. Despite the mandated decrease, Consumer Reports, which examined fuel economy data for every vehicle tested in the last five years, showed EPA estimates are still around 35 to 50% higher than the vehicle’s actual fuel efficiency (not surprising, considering EPA’s testing methods have not been updated in 20 years).

Even hybrids, those much-touted high-mileage vehicles, are not exempt from this problem.

Adding to the problem is the fact that in a country-by-country mileage comparison, the U.S. is consistently lowest. Greencarcongress.com reported two years ago that while the U.S.’s current average of new car fuel economy is nearly 30 mpg, European fuel consumption has fallen to approximately 43 mpg over the past three decades.

However, changes are in the works. The EPA recently revealed that, beginning in the 2007 model year, it will modify how it tests mileage (for the first time since 1985). Although each driver’s style determines how efficient his vehicle will be, it is expected that the new testing processes will be more in line with the average consumption.

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Mileage Myths - Autotrader