The auto companies have submitted to the EPA a plan for cars currently on dealer lots to be re-labeled with new window stickers reflecting the corrected mileage estimates. The mileage on most vehicle labels will be reduced by 1 to 2 mpg; the largest adjustment will be 6 mpg highway for the Kia Soul. The Sonata Hybrid will see a reduction of 1 mpg.

Both companies are putting in place a reimbursement program for affected current and former vehicle owners to cover the additional fuel costs associated with the fuel economy rating change. Customers will receive a personalized debit card that will reimburse them for their difference in the EPA combined fuel economy rating, based on the fuel price in their area and their own actual miles driven. In addition, as an acknowledgement of the inconvenience this may cause, the companies are adding an extra 15% to the reimbursement amount. Current owners will be able to refresh their debit card for as long as they own the vehicle. Prior owners of affected vehicles who have already sold their cars will also be reimbursed using the same formula.

[For more information about reimbursement and a complete list of eligible vehicles, customers can visit www.HyundaiMPGinfo.com and www.KiaMPGinfo.com.]

EPA’s audit testing occasionally uncovers individual vehicles whose label values are incorrect and requires that the manufacturer re-label the vehicle. This has happened twice since 2000. This is the first time where a large number of vehicles from the same manufacturer have deviated so significantly, EPA said.

At its National Vehicle and Fuel Emission Laboratory (NVFEL) in Ann Arbor, Mich., EPA routinely tests 150 to 200 vehicles per year—about 15% of the possible vehicle configurations—to ensure that their performance matches the mileage and emissions data required to be submitted to EPA by automakers.

This auditing helps to ensure that vehicles on the road meet tailpipe emission standards to protect public health and the environment and that all carmakers follow the same procedures for calculating mileage estimates. EPA conducts both random and targeted audits, based on factors such as consumer complaints.

EPA had received a number of consumer complaints about Hyundai mileage estimates. Through the agency’s ongoing audit program, staff experts at EPA’s NVFEL observed discrepancies between results from EPA testing of a MY2012 Hyundai Elantra and information provided to EPA by Hyundai. The agency expanded its investigation into data for other Hyundai and Kia vehicles, leading to the announcement.