Hyundai Motor America and Kia Motors America have overstated the fuel economies on the window stickers of some 900,000 automobiles from 13 models sold since late 2010, The Detroit News reports. An investigation by the U.S. Environmental Protection Agency (EPA) found that the results of its own tests did not always tie up with the companies' claimed mileages, and as a result Hyundai and Kia are lowering fuel economy estimates on what is reportedly a majority of current models.

"Given the importance of fuel efficiency to all of us, we're extremely sorry about these errors," Hyundai Motor America President and CEO John Krafcik told The Detroit News. "We're going to make this right."

Kia America's vice president for marketing and communications, Michael Sprague, told the News that Kia "really regrets deeply the errors" and that "we sincerely apologize to all our owners."

Hyundai had marketed four of its models on the basis that their fuel economies exceed the 40 mile-to-the-gallon benchmark, but the claim will no longer be applicable to the 2013 Accent, Veloster and Elantra, the News reports. The EPA investigation was prompted by consumer complaints about the Elantra in particular.

The companies, which are both part of the Hyundai Motor Group, have stated that owners of affected models will be reimbursed according to the shortfall in fuel economy, both on their mileage to date, and their mileage in the future. Both will reportedly add 15 percent to the dollar total once the shortfall has been calculated.

The issue appears to have come as a result of a change in the testing procedure introduced in 2010 to factor in road resistance. It's thought that cars validated before that time, which include models in the present ranges of the two manufacturers, are unaffected.

To put the scale of the mistake into some perspective, until these revelations sticker fuel economies have been reduced on only two models since 2000.

See The Detroit News for its full report.