Both the U.S. Environmental Protection Agency and National Highway Traffic Safety Admin. are expected to propose auto makers achieve a fleet-fuel economy of up to 60 mpg (3.9 L/100 km) when the two agencies release their first estimates on potential mandates for 2025 this month.

The federal offices, along with the California Air Resources Board, recently concluded a round of information gathering from auto makers, suppliers, state governments and other stakeholders.

The EPA and NHTSA were expected to use the feedback to draft a Notice of Intent to Issue a Proposed Rule for 2017-2025 fuel-economy and emissions standards.

Last year, the EPA and NHTSA issued a breakthrough set of rules, driving auto makers toward U.S. corporate average fuel economy of 34.1 mpg (6.9 L/100 km) by 2016. Each year, CAFE will average an increase of 4%, beginning in 2011 with '12 vehicles.

Together with tailpipe emissions targets, which offer auto makers credits for improving vehicle functions such as air conditioners, the U.S. fleet will reach 35.5 mpg (6.6 L/100 km) by 2016.

The notice due from the agencies by Sept. 30 represents the first step in the next round of CAFE rulemaking. The EPA and NHTSA then will seek comments on proposals within the notice before going back behind closed doors to craft a final rule, due as early as 12 months from now.

While the notice will not set a firm target for 2025, experts say 60 mpg likely will be among the proposed bogies.

“We've called on them to set 60 mpg by 2025,” says Brendan Bell, a Washington representative for the Union of Concerned Scientists' clean-vehicle program. The UCS played a key role in helping the government draft the 2016 rules, suggesting the 35.5 mpg mark ultimately decided upon.

The 2025 rule will cap upwards of three product cycles, allowing auto makers to leverage a host of coming technologies, such as downsized and turbocharged engines; lightweight materials; and hybrid, plug-in hybrid and full-electric propulsion systems.