WASHINGTON — Future generations of semi-trucks, school buses and large pickups will need to cut fuel consumption and emissions by 10 to 20 percent under first-ever fuel efficiency rules for trucks announced Monday by the Obama administration.

For the first time, the Environmental Protection Agency and the Transportation Department released proposed fuel economy requirements and reductions in tailpipe emissions for medium- and heavy-duty trucks, beginning with those sold in the 2014 model year and into the 2018 model year.

The proposal, which is expected to be finalized next summer, seeks a 20 percent cut in carbon dioxide emissions and fuel consumption from big rig combination tractors by 2018. Large tractor-trailers tend to be driven up to 150,000 miles a year, making them prime candidates for improved fuel efficiency.

Heavy duty pickup trucks, such as heavy-duty versions of the Ford F-Series, along with large vans would face separate gasoline and diesel truck standards phased in beginning in the 2014 model year. Vehicles running on gasoline would need to reach a 10 percent cut in fuel consumption and emissions by 2018 while diesel vehicles would need to hit 15 percent reductions by then.

So-called "vocational trucks" such as garbage trucks and transit and school buses would need to achieve a 10 percent reduction in fuel consumption and emissions by 2018.

The White House has sought stricter fuel economy standards across the nation's fleet as a way to reduce dependence on oil and cut greenhouse gas emissions tied to global warming. The fleet of new cars, pickup trucks and SUVs will need to reach 35.5 mpg by 2016, and the government is developing plans for future vehicle models that could push the standards to a range from 47 mpg to 62 mpg by 2025.

Medium-duty and heavy-duty trucks are much less fuel-efficient than conventional automobiles; the fleet of tractor-trailers typically get about 6 mpg to 7 mpg, while work trucks can achieve 10 to 11 mpg. While only representing 4 percent of the vehicles on the road, they use about 20 percent of the transportation fuel in the U.S.

EPA Administrator Lisa Jackson said "greater fuel economy will shrink fuel costs for small businesses that depend on pickups and heavy duty vehicles, shipping companies and cities and towns with fleets of these vehicles."

The improvements in fuel efficiency will come through a combination of more efficient engines, improved aerodynamics and better tires.

Bill Graves, president of the American Trucking Associations, said the proposal was "feasible and can be attained through technologies currently available to motor carriers."