The U.S. Environmental Protection Agency says Hyundai and Kia overstated the gas mileage on most of their models from the past three years.

The agency found the problems during an audit. It's making the companies retrofit window stickers to show lower mileage numbers.

Both companies say they're sorry for the errors and they'll reimburse customers for the difference between the mileage figures. They blame procedural mistakes and say they didn't intend to inflate the numbers.

The EPA says the companies will have to cut the gas mileage estimates by one to two miles per gallon on most models. One model's highway fuel economy was inflated by six miles per gallon.

The agency says the investigation is continuing. It wouldn't comment on whether the companies will be fined.