Hyundai and Kia are making adjustments to the official fuel consumption ratings of many of their 2011, 2012 and 2013 models.

As declared in a press release, following discussions with the U.S. Environmental Protection Agency (EPA), they are voluntarily adjusting the fuel economy ratings in the U.S. for approximately 900,000 units, or 35 percent of, 2011, 2012 and 2013 model year vehicles sold through Oct. 31, 2012.

On the Canadian side, approximately 172,000 units are affected, 130,000 for Hyundai Canada and 42,000 for Kia Canada.

No worldwide numbers have been revealed.

Hyundai and Kia declared jointly that the issue comes from procedural errors at the automakers’ joint testing operations in Korea; this led to “incorrect fuel economy ratings for select vehicle lines.”

Of course, the fuel economy ratings for vehicles currently in showrooms are being relabeled.

Following these changes, the 2012 Hyundai/Kia fleet fuel economy level is reduced in the U.S. by an average of 3 percent – from 27 to 26 mpg.

Kia Optima Hybrid and Hyundai Sonata Hybrid are touched by this adjustment.

For both the Optima Hybrid and Sonata Hybrid, the erroneous fuel economy label read 35 city / 40 highway / 37 combined; revised ratings are 34/39/36 respectively. Essentially, one mpg less in any situation.

To obtain proper information on reimbursement, U.S. customers can visit www.HyundaiMPGinfo.com and www.KiaMPGinfo.com; Canadian customers can visit www.HyundaiFuelConsumption.ca and www.KiaFuelConsumption.ca. These websites will be operational by end of day today, with the reimbursement process being operational by Monday.

The reimbursement will take into account the distance covered, the length of time the vehicle has been owned, as well as the local price of gas; Steve Kelleher, President and CEO of Hyundai Auto Canada Corp. added during a phone press conference that “there is a 15 percent amount added to compensate for the inconvenience.”

Both Kia and Hyundai said the compensation will be ongoing as long as vehicles touched by this process are in the hands of the original owner.

“Given the importance of fuel efficiency to all of us, we’re extremely sorry about these errors,” said John Krafcik, president and CEO of Hyundai Motor America. “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.”

“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 CEO, Kia Motors America and Kia Motors Manufacturing Georgia. “Our reimbursement program is intended to ensure that all affected Kia customers quickly receive fair compensation.”