Hyundai Motor Co and Kia Motor Corp admitted on Friday that they overstated the fuel economy on more than 1 million vehicles. The companies have jointly agreed to compensate people who have purchased cars with production years from 2011 through 2013 for the additional fuel costs according to Reuters.  

"Given the importance of fuel efficiency for all us, we are extremely sorry for these errors," said John Krafcik, head of Hyundai Motor America to Reuters. "When we say to Hyundai owners, 'We've got your back,' that's an assurance we don't take lightly.

The U.S. Environmental Protection Agency released a report this week stating they found errors in 13 Kia and Hyundai models. The investigation began after almost a dozen customers complained about their cars stating that there was a "gap between mileage ratings shown on the vehicle's Monroney stickers and what they actually got in real world use."

Over 900,000 vehicles sold in the U.S. through the end of October were affected by the changes according to the report. This amounts to over 35 percent of total cars sold in the U.S. this year.

The discovery took place during a routine audit by the EPA according to NBC News, and now the automakers could face government sanctions that could cost the companies millions of dollars in fines.

How was this problem allowed to happen in the first place?

A vital part of the tests that determine fuel economy includes measuring the resistance of a number of different systems. Such systems include how the specific car cuts through the air, how tires meet the rad, and how the vehicles moving parts work together according to The Detroit Press.  

Then you have to take into account how many miles are on the car, the condition of the tires, among other aspects. The last step includes determining "which kind of regulation and guidance procedures" they should follow.

"This test is extremely complicated and delicate and sensitive," said Sung Hwan Cho, who works for Hyndai-Kia to The Detroit Press. "We have been introducing many different models of vehicles to the market over the past three or four years, so we need to handle various types of models within limited amounts of time. So that's made us improve the efficiency of the test itself - how you efficiently make the test and generate the results. By doing that we added more steps."

The EPA has yet to determine how they plan on handling testing standards by car companies or if they will just audit more cars. The organization audits over 15 percent of vehicles released each year.  

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

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