Korean brands Hyundai and Kia will restate the U.S. fuel economy rating for many of their cars, after the Environmental Protection Agency determines the OEMs’ testing methodology is flawed.

The overstated fuel-economy ratings were for roughly 900,000 units, or 35%, of the 2.7 million ’11-’13 model vehicles the two auto makers sold through October.

“EPA had received a number of consumer complaints about Hyundai mileage estimates,” the agency says in a press release.

The EPA says experts at its National Vehicle and Fuel Emission Laboratory in Ann Arbor, MI, noticed discrepancies between results from EPA testing of a ’12 Hyundai Elantra, during a periodic audit program, and information provided to the agency by Hyundai.

The agency expanded its investigation into data for other Hyundai and Kia vehicles, leading to today’s announcement. The models involved include the Hyundai Accent, Azera Elantra, Genesis, Tucson, Santa Fe and Veloster and the Kia Rio, Sorento, Sportage and Soul.

Hyundai blames erroneous testing for road resistance for the fuel-economy misstatements. “These were just honest procedure errors in a very complex testing process,” says Sung Hwan Cho, president of the Hyundai-Kia America Technical Center.

Because of the high volume of models released by the auto maker in recent years, he says Hyundai and Kia have been adding steps to the EPA process to improve testing efficiency and reduce variations in procedures.

For instance, the team that measures fuel economy for both brands was testing for tire-rolling resistance on a test track and not on public roads.

“The tire is a very important part which can affect the rolling resistance of the vehicle,” Cho says a conference call today with media. “So how you prepare the tire is very important. What we did is, instead of using public roads to break in the tire, we used the test track, a very well-defined condition.”

Some Hyundai and Kia models will be dinged worse than others. The Kia Soul, for example, will see its highway fuel economy fall 6 mpg (2.6 km/L) in the 2.0L 4-cyl., 6-speed automatic transmission model, from 34 to 28 mpg (6.9 L/100 km to 8.4 L/100 km).

The controversial Hyundai Elantra compact car’s economy drops as much as 2 mpg (0.9 km/L) in certain ’13 models, such as the GT hatchback. Elantra owners have complained loudly, with some filing lawsuits, that they have been unable to achieve the auto maker’s stated fuel economy of 40 mpg (5.9 L/100 km) highway.

John Krafcik, president and CEO of Hyundai Motor America, does not directly address Hyundai’s position on any lawsuits regarding the misstated fuel economy. “All we can do is let the lawsuits proceed,” he says during the conference call. “We think this reimbursement program we put in place is a very good program.”

Krafcik refers to debit cards the two auto makers say they plan to issue current and past owners of affected models.

The debit card “will reimburse (customers) for their difference in the EPA combined fuel economy rating based on the fuel price in their area and their own actual miles driven,” Hyundai-Kia says. A 15% bonus also will be added to the cards for any inconvenience to car buyers the situation may cause.

For example, a Florida owner of a Hyundai or Kia model seeing a 1 mpg (0.4 L/100 km/L1) reduction in combined fuel economy, and who has driven 15,000 miles (24,140 km) this year, could receive a debit card for $88.03, Kia Vice President of Marketing Michael Sprague says.

Current owners can refresh their debit card as needed by going to their dealer and having their mileage checked. Past owners of affected models will receive a debit card just once, with the reimbursement amount based on the number of miles listed on the bill of sale for the vehicle.

Hyundai and Kia each have set up websites where customers can sign up for compensation. Dealers have been notified of the program and are receiving revised window stickers today.

Hyundai-Kia will launch an ad campaign that includes placing a letter in Sunday editions of major U.S. newspapers, as well as directly contacting current or past owners by mail or online.

Both Hyundai’s Krafcik and Kia’s Sprague express regret over the situation, but neither says his company will pull back on fuel-economy messaging in advertising. Hyundai has been especially aggressive in touting the number of 40-mpg models it offers.

“Even with these adjustments, our cars will continue to be in an industry-leading position in fuel efficiency,” Krafcik says. For Hyundai-Kia, combined, this amounts to a 1-mpg change to the brands’ fleet average for the ’12 model year – from 27 to 26 mpg (8.7 to 9.0 L/100 km).

Krafcik says Hyundai and Kia do not yet know what the cost of the reimbursement program will be for the Korean parent, but says it will be “certainly in the millions, no question.” It is undetermined whether the EPA will fine the auto makers.

Hyundai has long insisted its fuel-economy numbers were achievable so long as customers drove in a fuel-efficient manner, such as avoiding jack-rabbit starts and stops. It cited third-party evidence, such as media tests, to bolster its position.

WardsAuto editors have had mixed experiences testing Hyundai and Kia models. A test drive last year of the 1.6L Kia Soul Eco with a 6-speed automatic and stop/start technology saw some editors surpassing the 32 mpg (7.4 L/100 km) average. However, one editor found the Elantra fell short of the 40-mpg highway fuel economy rating.

The 1.6L Soul Eco will see its fuel economy restated to a combined 28 mpg (8.4 L/100 km) for ’12 and ’13 models.

cschweinsberg@wardsauto.com