Yesterday Hyundai and Kia admitted to overstating the estimated fuel economy of about 900,000 cars by way of window stickers since 2010, after the U.S. Environmental Protection Agency started poking around. That’s a major no-no, so the companies will now have to shell out millions of dollars to customers to make up for those faulty claims.

The Detroit News reports that the EPA found discrepancies in the fuel economy estimates on most 2012-2013 models when it tested the vehicles on its own, and then compared those results to what the company’s data showed.

Selling cars based on high gas-mileage is a big score usually for car companies, as customers want to spend less on gas and often do some good for the environment, too. The EPA said the mpg sticker values had been reduced on only two vehicles since 2000.

Hyundai will be taking back its claim that it has four models that get 40 mpg on the highway, as the 2013 Accent, Veloster and Elantra will fall to 37 or 38 mpg.

In an interview with The Detroit News, executives from the two companies in the U.S. apologized and promised to make it up to their customers.

“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.”

He said the problem was all due to “procedural errors” in the fuel economy testing but that the company has sorted that out and made sure its testing method and the way the EPA does things are now in line.

As for Kia:

Michael Sprague, Kia America’s executive vice president for marketing and communications, said the company “really regrets deeply the errors and … we sincerely apologize to all our owners.”

Instead of having an average of 27 mpg for the two companies’ combined fleet, it’ll be about 26 mpg economy on average, or a loss of around 3%.

The vehicles impacted come to 35% of the 2011-2013 vehicles sold through October, a total of around 900,000 vehicles. The majority of those will have mileage estimates that fall by 1 mpg, with around 240,000 more seeing mileage estimates drop by 2 mpg and 80,000 falling by 3 to 4 mpg.

New window stickers are being printed out already, and as Hyundai’s Krafcik said, “We should have this done in a matter of days.”

So how will you be reimbursed if your vehicle is affected? Bring your car to the dealer to get its odometer checked. Dealers will then calculate how much you would’ve saved if your car had actually achieved the gas mileage that was promised, and 15% of that dollar total will be added in. Car owners will receive a debit card in the mail, and will continue to be reimbursed for as long as they own the car.

Hyundai, Kia misstate mileage claims [The Detroit News]

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