Automakers see merit in an EPA proposal that would require them to road test vehicles to verify mileage claims, but they are unclear on how the tests would be handled.
The proposal under consideration would require automakers to perform "supplemental test track audits" of light vehicles, along with laboratory testing. The move follows the recent restatement of EPA ratings on several vehicles by Hyundai, Kia and Ford.
But tests must account for variables such as wind speed, temperature and pavement conditions.
"I'd love to see a move towards more realistic and more real-world testing," AutoPacific analyst Dave Sullivan said.
But he added: "There are a lot of different variables. It's a good idea, but I'm not sure what implementation will be like."
The proposed road test would make real-world driving trials more rigorous and reflect air resistance and rolling friction on a test track rather than in a test lab, according to The Wall Street Journal, which first reported the development.
Those factors can affect fuel economy considerably, especially in hybrids.
The difference between a driver's actual mileage and what a vehicle is rated is among the most frequent consumer complaints and questions posed to the agency.
"Augmenting EPA's existing pre-production procedures with post production audits of real world factors will help further ensure that the data used in EPA labels accurately reflect the vehicle consumers find on dealer lots," the agency said in a statement.
But the EPA said it is too early to speculate what form the tests would take. It said the proposal would be subject to a public notice, comment and rulemaking process.
Ford amended mileage estimates on several hybrids last month, and Hyundai is being sued by consumers in South Korea over accusations it overstated the fuel efficiency of the Santa Fe crossover.
In 2012, Hyundai and affiliate Kia apologized and compensated owners for overstating mileage estimates on light-vehicle models sold in the United States.
"It is too early to know precisely what it is the EPA is looking for," Hyundai spokesman Chris Hosford said. "There are so many variables to keep track of, so it is too early to comment at this time."
A Ford spokesman said: "It is an important effort by the agency to address the inconsistency across the industry."
A GM spokeswoman said the automaker is "comfortable" with the EPA initiative.
"We already perform our own internal audits and testing similar to what is referenced as 'supplemental test track audits,' and have done so for many years."
The EPA, as The Journal noted, does not evaluate every new light vehicle to verify mileage claims. Most fuel economy tests are performed by automakers, and the information is later shared with and reviewed by the agency.
The EPA last adjusted fuel efficiency testing in 2008, which narrowed the gap between window sticker numbers and what owners experienced in real-world driving.