Mercedes-Benz was the worst offender, as the Daimler division on average overstated its vehicles' fuel economy by 48 percent, said the study, which used data from International Council on Clean Transportation (ICCT). That automaker isn't exactly a world-beater for fuel economy in the US. In fact, two of its models, the Mercedes-Benz G 63 AMG and the G550 (pictured), showed up on the American Council for an Energy-Efficient Economy's (ACEEE) annual list of cars that are "meanest" to the environment.
"The information provided by manufacturers on fuel consumption is based on the NEDC test cycle, which is prescribed by law and conducted in a laboratory. Since real driving conditions generally deviate from conditions in the laboratory, the consumption figures can also deviate from the standardized figures. Since T&E has unfortunately not published the test conditions used in its study, it is not possible to provide a useful analysis of the test results," Mercedes-Benz wrote in an e-mailed statement to Autoblog. "The data pool used also does not allow a thorough scientific assessment. Mercedes-Benz emphatically supports the introduction of the WLTP (Worldwide Harmonized Light Vehicles Test Procedure). This is supposed to replace the NEDC with the aim of bringing the rated fuel consumption and the actual consumption closer together. We also actively support the dialog between industry (ACEA) and the authorities, and are in regular contact with the EPA and the ARB in the US."
The ICCT is the group that helped spur the investigation that led to the Volkswagen diesel-emissions scandal we're all still talking about, so it shouldn't expect Christmas cards from the largest German automakers this year. This new study came from data taken from about 600,000 cars. That's a lot of funky air floating over Rome. The overstatements were pretty widespread, too. The fuel-economy of the BMW 5 Series was overstated by almost 50 percent, as was the Peugeot 308's.