Hyundai and Kia overstated the estimated fuel economy on more than 900,000 U.S. vehicles sold over the past two years and will compensate owners for the faulty claims, a newspaper reported.

In addition, the South Korean partners will lower the fuel-economy estimates on most of their 2012 and 2013 models, the Detroit News said. The move follows an investigation by the U.S. Environmental Protection Agency, which found discrepancies between its own test results and company figures.

The newspaper said Hyundai Motor America CEO John Krafcik attributed the problem to "procedural errors" in the automaker's own testing. He said Hyundai has identified the source of discrepancies between its testing method and the EPA's recommended approach.

"Given the importance of fuel efficiency to all of us, we're extremely sorry about these errors," Krafcik said. "We're going to make this right."

Michael Sprague, executive vice president for marketing at Kia Motors America, also apologized in an interview with the newspaper.

The newspaper said Hyundai will retract its claim that it leads the industry with four models that get 40 miles per gallon on the highway. According to EPA records obtained by the paper, the estimated highway mileages of the 2013 Accent, Veloster and Elantra will fall to 37 or 38 mpg.

The combined average fuel economy for Hyundai and Kia models will fall to 26 mpg from 27 mpg for the 2012 model year, the News said.

The newspaper said Hyundai and Kia dealers will check odomoters to determine how much owners might have saved had they achieved the stated gas mileage.

The companies will then add 15 percent to that dollar total and send debit cards to owners for as long as they own their cars.

Sprague told the newspaper that an owner who drove 15,000 miles in Florida this year in a car with an overstated fuel economy of 1 mpg would get a refund of about $88.

New window stickers with the adjusted fuel-economy figures should be applied to unsold Hyundais and Kias within days, Krafcik told the newspaper.

Hyundai and Kia are owned by the same parent company and share factories, research, platforms and powertrains across many car and light-truck models.

However, they market and sell models through separate dealership channels.

The Associated Press said the EPA received about a dozen complaints from consumers that the mileage on their 2012 Hyundai Elantra compact cars didn't match the numbers on the window stickers.

"Consumers rely on the window sticker to help make informed choices about the cars they buy," Gina McCarthy, assistant administrator of the EPA's air-quality office, said in a statement cited by the AP. "EPA's investigation will help protect consumers and ensure a level playing field among automakers."

-- Automotive News

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