Transportation Department Secretary Mary Peters outlined the plan on Earth Day, setting a schedule that was more aggressive than initially expected by industry officials.
Peters said the proposal was "an aggressive but achievable standard. I think we've got something that is going to significantly save fuel and help clean our air."
The plan responds to a new energy law pushed by Congress and signed by President Bush that requires the nation's new cars and trucks, taken as a collective average, to meet 35 mpg by 2020.
New cars and trucks will have to meet a fleetwide average of 31.6 mpg by 2015, or about a 4.5 percent annual increase from 2011 to 2015.
The fleetwide average doesn't mean that all vehicles have to achieve the target efficiency - larger vehicles will not be as fuel efficient as smaller ones - only that collectively passenger vehicles average that figure.
The plan is expected to save 54.7 billion gallons of oil and reduce carbon dioxide emissions by 521 million metric tons over the life of the new vehicles built between 2011-2015. It will add an average cost of $650 per passenger car and $979 per truck by 2015.
The proposal is expected to be finalized by the end of President Bush's term in office.
Automakers opposed increases to the regulations in previous years, but supported a compromise version of the legislation in Congress amid rising gasoline prices and concerns about global warming.
The regulations would require the industry to implement more than half of the fuel-efficiency requirements by 2015 and push them to build more gas-electric hybrid cars, diesel-powered trucks and SUVs and advances such as plug-in hybrids and electric vehicles.
"These numbers are very challenging. They will stretch the industry to innovate in ways they haven't had to do in the past and will continue to set us on a course to significantly reduce greenhouse gas emissions from new autos," said Charles Territo, a spokesman for the Alliance of Automobile Manufacturers, which represents General Motors Corp., Toyota Motor Corp., Ford Motor Co. and others.
Amid rising gasoline prices and concerns of global warming, Congress sought the tougher standards, requiring the nation's fleet of new vehicles to increase its efficiency by 10 mpg from its current average of 25 mpg, or a 40 percent increase.
The new law represented the first major changes to the auto mileage rules in three decades.
The fleet of new passenger cars is currently required to meet a 27.5 mpg average, while sport utility vehicles, pickup trucks and vans must hit a target of 22.5 mpg.
Members of Congress and environmental groups have pushed for higher standards, arguing that requiring vehicles to become more efficient would help reduce greenhouse gas emissions and the nation's dependence upon imported oil.
Democrats have said the fuel economy requirements will save motorists $700 to $1,000 a year in fuel costs and reduce oil demand by 1.1 million barrels a day when the more fuel-efficient vehicles are in wide use on the road.