The EPA said it had fielded customer complaints the mileage estimates on Hyundai vehicles, which prompted them to audit the MY2012 Hyundai Elantra. Finding discrepancies between tests and claims made by the company prompted further testing, the EPA said.
"Customers rely on the window sticker to help make informed choices," said Gina McCarthy, assistant administrator for the agency's Office of Air and Radiation.
"EPA's investigation will help protect consumers and ensure a level playing field among automakers," she said.
The EPA tests 150 to 200 vehicles a year to ensure emission control and mileage claims are accurate and consistently handled by various companies.
The agency said Hyundai Motor America and Kia Motors America would lower estimates "for a majority of their model year 2012 and 2013 models" after tests found them to be inaccurate.
The automakers will lower estimates by 1 and 2 mpg for most brands, and as much as 6 mpg for the Kia Soul's highway estimate.