DETROIT — The South Korean carmakers Hyundai and Kia built their brands around the idea that their cars got better gas mileage than competitors, promoting that fact in ads that often took swipes at less efficient rivals.

But on Friday, the companies admitted that they had overstated the fuel economy of 900,000 vehicles sold in the United States over the last two years — about one-third of the vehicles they sold during that period.

Hyundai and Kia, which are both owned by the Hyundai Motor Group, said they would begin a broad effort to reimburse consumers and restate mileage estimates for the affected models.

The admission followed an investigation by the Environmental Protection Agency into consumer complaints that their cars were underperforming the official mileage estimates on the window stickers of new Hyundai and Kia vehicles. While few drivers achieve the stickered mileage levels under real-world conditions, the government requires automakers to conduct standardized tests to calculate the figures so that buyers can more easily compare the fuel efficiency of various models.