EPA finds Mercedes C-Class fuel economy figures are off

Through the years, Consumer Reports has cited numerous discrepancies between our fuel economy test results and the official EPA-sanctioned numbers on window stickers. Even as the government ratings system has improved over time, there have been significant variations on occasion that have exposed automakers overstating claims. Back in August, the EPA announced that it would begin auditing the fuel economy tests that automakers conduct in an effort to reduce cheating and make labels more accurate for consumers. Now those audits have just snared their first vehicle, the last-generation Mercedes-Benz C-Class.

The 2013 and 2014 Mercedes-Benz C300 4Matic carried fuel economy estimates of 20 mpg city, 29 mpg highway, and 23 mpg combined. In follow-up analysis, the EPA found the cars actually got 1 mpg less in all three tests. C300s certified as flex-fuel vehicles (which can burn E85 ethanol in place of gasoline) were rated at 20 mpg city, 27 highway, and 22 combined. Those cars lost 1 mpg in the city validation test.

Consumer Reports found the rear-drive C300 returned 21 mpg overall—a notable shortfall from the advertised fuel economy. These new adjustments now put the official figure closer to reality. (Learn more about how Consumer Reports tests cars.)

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Mercedes-Benz will now restate the fuel economy labels for both of these models, although the impact is fairly limited. As Mercedes points out, this represents one of 70 model variations from their portfolio. Since the 2015 model year brings an all-new C-Class, the company states that only about 500 remaining 2014 C300 4Matics will need revised window stickers.

EPA began auditing tests conducted by automakers after consumers reported, and Consumer Reports testing confirmed, that certain cars couldn't match their official window-sticker mpg estimates. The most frequent offenders were hybrids and cars with small, turbocharged engines.

Ford restated fuel economy estimates for six models in July after finding the cars didn't get the fuel economy that their estimates indicated. In 2012, Hyundai restated the fuel economy of 12 models, including three Kias. In those cases, the automakers also issued credits to consumers to compensate them for the extra fuel they used.

We applaud the EPA continuing its efforts to improve fuel economy ratings, and thereby putting added pressure on the auto industry to be accurate, while also making the marketplace better for consumers. Based on other discrepancies we've seen in testing, this may be just the tip of an iceberg.

—Eric Evarts