Two federal agencies proposed today the first national greenhouse gas and fuel efficiency standards for heavy-duty trucks and buses, including rules intended to boost fuel consumption for smaller diesel vehicles by 15% and for gasoline-powered vehicles by 10% as well as varying standards for bigger load-haulers.

Those standards, plus different rules for combination tractors (the kind of tractor-trailers usually used for long-haul deliveries) and vocational vehicles (such as school buses) are intended to reduce greenhouse gas (GHG) emissions by about 250 million metric tons and save 500 million barrels of oil over the program's first five years, the Environmental Protection Agency (EPA) and the Department of Transportation's National Highway Traffic Safety Administration (NHTSA) said in a joint news release.

NHTSA standards would be voluntary for the 2014 and 2015 model years and then become mandatory on a phased-in basis from the 2016 through the 2018 or 2019 model years.

EPA and the Transportation Department also issued a proposed rule that will be open for comment for 60 days once it's published in the Federal Register. EPA and NHTSA also will hold joint public hearings on Nov. 15 in Chicago and on Nov. 18 in Cambridge, Mass.

:"These new standards are another step in our work to develop a new generation of clean, fuel-efficient American vehicles that will improve our environment and strengthen our economy," EPA Administrator Lisa P. Jackson said in the joint news release. "In addition to cutting greenhouse gas pollution, 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 agencies estimated the program would generate $41 billion in net benefits over the lifetimes of the vehicles produced between 2014 and 2018. For instance, it said the fuel efficiency gains in a semi-trailer truck would pay for the increased costs of technology in under a year, and yield as much as $74,000 over the truck's useful life." Those technology costs include widespread use of aerodynamic improvements and tire rolling resistance, as well as engine and transmission upgrades, they said.

Vehicles with lower annual miles would typically experience longer payback periods, up to four or five years, but would still reap cost-savings," the agencies said.

The proposal generally defines heavy-duty trucks as those with gross vehicle weights of at least 8,500 up to and above 33,000 pounds.

The proposed reductions in GHG emissions and improvements in fuel mileage vary by vehicle type. For example, the proposed mileage standards for heavy-duty tractor-trailers vary by class as well as roof height, going from a minimum of 6.3 miles per gallon to a top requirement of 11.4.

For more from the EPA on the proposal, see its information page. NHTSA also has published its own information page.