Hyundai and Kia will compensate 900,000 owners for the exaggerated mileage claims.

Following an investigation by the Environmental Protection Agency, Hyundai and Kia have admitted to misstating the fuel economy of about 900,000 vehicles built since 2010. The company will reduce the fuel economy estimates on many of its 2012-13 models and plans to compensate owners for the exaggerated claims.

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

In all, about 35 percent of the 2011-13 models Hyundai and Kia sold through October will have their mileage estimates reduced. 580,000 vehicles will see their ratings drop by 1 mpg, 240,000 will fall 2 mpg while the remaining 80,000 will drop 3 to 4 mpg, according to a Detroit News report.

The 2013 Hyundai Elantra, Accent and Veloster will likely see their EPA highway ratings fall from the magic (and aggressively advertised) 40-mpg mark to either 37 or 38 mpg.

Procedural Errors to Blame

After the EPA's probe found discrepancies between the agency's own results and Hyundai's data, the automaker states it discovered "procedural errors" in its coastdown testing. Hyundai says that coastdown testing "simulates aerodynamic drag, tire rolling resistance and drivetrain frictional losses and provides the technical data used to program the test dynamometers that generate EPA fuel economy ratings."

"I sincerely apologize to all affected Hyundai and Kia customers, and I regret these errors occurred," said Dr. W. C. Yang, chief technology officer of Hyundai/Kia research and development. "Following up on the EPA's audit results, we have taken immediate action to make the necessary rating changes and process corrections."

Hyundai is currently printing up new window stickers with the revised EPA ratings and expects to have them applied within days.

Reimbursement Program

Hyundai and Kia will compensate owners for the money they would have saved if their vehicles had achieved the advertised fuel economy, based on local fuel prices and miles driven.

The automakers will give owners debit cards and add an extra 15 percent to the dollar total in further recompense. The reimbursement will continue for as long as customers own the vehicles.