Hyundai and Kia are adjusting the fuel economy ratings for approximately 900,000, or 35%, of 2011-2013 model-year vehicles sold through Oct. 31, 2012.

The companies stated they are taking this voluntary effort after discussions with the U.S. Environmental Protection Agency (EPA). Hyundai and Kia said procedural errors at the automakers’ joint testing operations in Korea led to incorrect fuel economy ratings.

The companies added that this adjustment will reduce the fuel economy of the 2012 Hyundai/Kia fleet by an average of 3%, from 27 mpg to 26.

Hyundai models affected include:

• 2011 Elantra and Sonata HEV

• 2012 Accent, Azera, Elantra, Genesis, Sonata HEV, Tucson, Tucson 4WD and Veloster.

• 2013 Accent, Azera, Elantra, Elantra Coupe, Elantra GT, Genesis, Santa Fe Sport 2WD and 4WD, the Tucson 2WD and 4WD, and the Veloster.

• Kia models affected include the 2011-2012 Optima HEV, and the 2012-2013 Rio, Sorrento, Soul, Soul ECO and Sportage.

To compensate for this error, the companies are setting up a reimbursement program for affected vehicle owners to cover the additional cost of fuel due to the fuel economy rating change. The program will provide customers with a personalized debit card that will reimburse them for the difference between the previous and adjusted combined EPA fuel economy numbers, and will take into account the fuel price in the customer’s area and actual miles driven.

The companies added they will add 15% to the reimbursement amount to compensate for the inconvenience caused by the fuel economy discrepancy.

Further explaining the procedural errors, the companies stated that the testing process with the errors is called “coastdown.”Coastdown simulates aerodynamic drag, tire rolling resistance and fuel economy loss due to friction in the drivetrain. This testing process provides the EPA with the data used to program the test dynamometers the EPA uses to generate its fuel economy ratings.

You can visit Hyundai’s website here, and Kia’s here, for more information about the models affected and the reimbursement program.