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How Does the EPA Reach Its Fuel Economy Numbers?

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author photo by Doug DeMuro June 2015

For most drivers, fuel economy figures are really important. They can make or break whether you buy a certain car or whether you pass on it for something less thirsty. But have you ever stopped to wonder how these figures are calculated? Today, we're telling you exactly how the Environmental Protection Agency (EPA) manages to provide accurate gas-mileage numbers for every single car on the market.

Most Cars Aren't Tested...

Possibly the most interesting thing about the EPA gas-mileage testing procedure is what the government doesn't test: namely, most new cars.

That's right: Although the EPA provides fuel economy figures for virtually every new car on sale today, the agency itself doesn't actually do most of the testing. Instead, the EPA only tests around 15 percent of today's latest cars, trusting automobile manufacturers to self-report the rest.

Although this may seem like it could be fraught with consequences -- automakers beefing up their numbers for good publicity, for example -- the EPA's testing generally keeps car brands honest. After all, if an automaker were to overestimate its fuel economy figures and the EPA were to select that car for testing, it could be a public-relations disaster. As a result, automakers are only rarely caught inflating their gas-mileage numbers.

...But Some Are

As for the 15 percent of cars that are tested by the EPA, the agency has a surprisingly straightforward testing procedure. No, the vehicles aren't brought out on the open road to see what they can do. Too many variables -- such as weather conditions, road surfaces and unplanned detours -- could diminish the accuracy of the agency's testing procedure.

Instead, cars are tested in possibly the most consistent way possible: in a laboratory and on a dynamometer, also called a "rolling road," where the agency's experienced drivers follow the same prescribed road pattern for each vehicle. There's an 11-mile city test, designed to mimic driving in a high-traffic city, along with a longer highway test that gets up to 80 miles per hour. Although a few extra steps are necessary to reach final calculations, such as using portions from each test rather than figures from the entire test, this is the basic gist of the EPA's city and highway gas-mileage numbers.

Real-World Accurate?

In years past, the EPA's fuel economy numbers didn't quite match up to real-world driving -- largely because the agency's highway test still followed a speed limit of 55 mph long after the federal government had allowed states to increase their limits. The modern test is far more accurate thanks to higher speeds and even a period where the vehicle's air-conditioning system is used.

As a result, we think today's fuel economy figures are mostly accurate, and they're certainly a lot better than they once were. But the phrase "your mileage may vary" exists for a reason -- so you might want to take a long, careful test drive dedicated to measuring gas mileage if you consider miles per gallon to be a top priority.

This image is a stock photo and is not an exact representation of any vehicle offered for sale. Advertised vehicles of this model may have styling, trim levels, colors and optional equipment that differ from the stock photo.
How Does the EPA Reach Its Fuel Economy Numbers? - Autotrader