Hyundai Motor and its affiliate Kia Motors on Thursday unveiled a new road map for improving the fuel efficiency of their new cars by 25 percent on average by 2020.

As part of the broad efforts, the two Korean carmakers will be pouring resources into developing new engines and transmissions and trimming car weight, officials said.

The companies did not reveal the estimated costs involved in the fuel efficiency overhaul.

“A special task force has been working on the issue since early this year after chairman Chung Mong-koo stressed competitiveness in fuel economy,” said a Hyundai spokesperson.

“We will preemptively meet the more stringent global fuel-economy requirements.”

Under the new fuel-economy initiative, Hyundai and Kia said they would replace about 70 percent of their current engine lineups shared across models with new ones by 2020.

Starting with gasoline engines, their turbo-charged versions and diesel engines will be developed in phases. The new gasoline engines are expected to improve the cars’ fuel economy by 11 to 13 percent, while diesel engines will improve fuel economy by 16 to 18 percent.

In partnership with Hyundai Steel, the group’s steelmaking unit, the carmakers will continue to increase the use of high-strength steel, a stronger but lighter alternative to conventional steel.

The rate of adoption of high-strength steel is currently about 52 percent for new cars like the Hyundai Sonata and Genesis and the Kia Sorento. By 2020, they plan to elevate the figure to 48 to 62 percent.

“We will reduce car weight by more than 5 percent on average through the use of high-strength steel and other lightweight materials,” the official said.

The new plan comes as consumers and analysts have been increasingly challenging the accuracy of fuel-economy stickers on Hyundai and Kia vehicles.

The Korean conglomerate built its brands around the idea that their vehicles had better mileage than their competitors. But the claims have been tarnished in recent years after they were found to have overstated the fuel efficiencies of several models.

In 2012, Hyundai and Kia admitted that they had misstated the fuel ratings of about 1.2 million vehicles sold in the U.S., which led to a large-scale compensation program for customers for their extra fuel consumption.

The companies said that the misstatement of fuel mileage was inadvertent and that they did not intentionally mislead customers.

The U.S. Environmental Protection Agency responded on Tuesday by slapping Hyundai and Kia with a fine of $100 million, along with carbon credit forfeitures valued at more than $200 million.

Similar reimbursements have also taken place in Korea. In August this year, Hyundai announced it would reimburse some 140,000 Santa Fe SUV owners here for the false fuel ratings.

By Lee Ji-yoon (jylee@heraldcorp.com)