South Korean automakers Hyundai Motor America and Kia Motors America overstated the fuel economy on nearly 1 million late-model vehicles and will issue owners special debit cards to reimburse the extra money they are paying for fuel.
The error was announced Friday by the U.S. Environmental Protection Agency.
The mileage on most vehicle labels will be reduced by 1 to 2 miles per gallon, and the largest adjustment will be a 6-mpg difference for the Kia Soul's highway rating, federal regulators said.
The Korean automakers, which share automotive components and testing, said they had overstated fuel economy ratings for about 900,000 vehicles, or 35 percent of the 2011-13 model year vehicles sold through Wednesday.
They blamed "procedural errors" at joint testing operations in Korea for the problem.
The automakers will issue personalized debit cards to owners of the vehicles. The cards will reimburse the customers for the shortfall in the companies' mileage claims and what the EPA has found is the correct number for combined city and highway driving fuel economy rating.
The payment will be based on the fuel price in the region where owners live and on the miles they have driven. The cards also will include a 15 percent bonus to make up for inconvenience to owners.
Hyundai and Kia will refresh the cards to continue to make up for the cost of the fuel economy error for as long as the owners of the vehicles have the cars.