Orange County’s two largest automakers will lower the fuel economy estimates on most of their 2012 and 2013 model-year vehicles.

The company is offering affected vehicle owners a debit card good through the length of their vehicle ownership that will compensate for the additional fuel costs. The reimbursements will be based on area gas prices and miles driven.

Tests conducted by the U.S. Environmental Protection Agency found discrepancies between the agency’s findings and the numbers provided by Costa Mesa-based Hyundai Motor America Inc. and Irvine-based Kia Motors America Inc. on their vehicles’ fuel performance.

The two share a parent in South Korea-based Hyundai Motor Co. and operate under the Hyundai Kia Automotive Group.

The EPA’s announcement on Friday means a change of one to two miles per gallon on most vehicle display stickers that consumers use when shopping for cars. Kia’s Soul will see the largest change of six mpg.

The EPA’s investigation was triggered by consumer complaints on Hyundai vehicles. The EPA tested a 2012 Hyundai Elantra after noting discrepancies between its numbers and that of Hyundai’s, prompting a broader investigation into additional Hyundai and Kia vehicles.

“Given the importance of fuel efficiency to all of us, we’re extremely sorry about these errors,” said Hyundai Motor America Chief Executive John Krafcik in a statement. “When we say to Hyundai owners, ‘We’ve got your back,’ that’s an assurance we don’t take lightly. We’re going to make this right for everyone, and we’ll be more driven than ever to ensure our vehicles deliver outstanding fuel economy.”

The two companies estimate some 900,000 vehicles will be affected by the EPA’s findings. That’s about 35% of the companies’ 2011 through 2013 model year vehicles sold.

Hyundai and Kia attributed the discrepancies to “procedural errors at the automakers’ joint testing operations in Korea.”

“As a customer-focused organization, we are fully committed to providing consumers with complete and accurate information, and deeply regret the errors were made,” said Byung Mo Ahn, group president and Kia Motors America and Kia Motors Manufacturing Georgia chief executive, in a statement. “Our reimbursement program is intended to ensure that all affected Kia customers quickly receive fair compensation.”