The U.S. government's testing underestimates how much fuel cars will burn on the road, and the problem has gotten worse, according to a study released on Thursday.
The release of the study funded by the Department of Energy's Oak Ridge National Laboratory comes as regulators in the United States, Canada and Europe tighten their scrutiny of how cars perform in real-world conditions.
That comes after Volkswagen admitted last month to equipping its diesel cars with software that made the cars run clean in laboratory tests in order to hide actual emissions.
The gap between the better performance of cars in testing by regulators and the lower fuel economy drivers experience has been widely known. But a wider gap could jeopardize the United States from reaching its targets for reducing carbon emissions, according to the study by researchers at self reported the University of Tennessee and Oak Ridge
Fuel economy measurements used to certify compliance with federal regulations overestimated engine efficiency by roughly 15 percent for much of the 1990s and 2000s, research found.
But the Oak Ridge study found that driver-cited fuel economy was 25 percent below government estimates in 2013 and about 22 percent lower this year and last.
"The important thing is to see that the gap has been increasing by model year. We have to keep track of it and monitor to see that it doesn't increase," said David Greene, a senior fellow at the University of Tennessee, Knoxville, and one of the researchers.
The report analyzed approximately 75,000 individual fuel economy estimates reported by drivers to the EPA online.
Responding to the report, the EPA said that while its "testing cannot and does not purport to reflect national average driving behavior, weather, and traffic conditions," its fuel economy label on cars accounts for those variables.
"Since 2008 average fuel economy as reported by drivers has been very closely aligned with the fuel economy labels," said Christie St. Clair, a spokeswoman with the EPA.
The VW scandal was unearthed by researchers at the U.S.-based nonprofit International Council on Clean Transportation, which hired researchers at a West Virginia University lab.
A 2013 study by the ICCT found an even wider gap between testing and real world carbon emissions. That study found the gap had increased from 8 percent in 2001 to 38 percent in 2013. The study concluded that up to half of that gap could be the result of manufacturers designing vehicles that would perform better in lab testing.
The EPA said last week that it would step up its emissions testing on all kinds of vehicles in road driving conditions after the VW admission of cheating on emissions testing.