WASHINGTON — The next generation of new cars and trucks will need to meet a fleet average of 31.6 miles per gallon by 2015, the Bush administration proposed yesterday, seeking more fuel-efficient vehicles in the face of high gasoline prices and concerns over global warming.

The secretary of Transportation, Mary Peters, outlined the plan on Earth Day, setting a schedule that was more aggressive than initially expected by the auto industry. It responds to a new energy law that requires new cars and trucks, taken as a collective average, to meet 35 mpg by 2020.

“This proposal is going to help us all breathe a little easier by reducing carbon dioxide emissions from tailpipes, cutting fuel consumption and making driving a little more affordable,” Ms. Peters said. New cars and trucks will have to meet a fleet-wide average of 31.6 mpg by 2015, or about a 4.5% annual increase from 2011 to 2015. In 2015, passenger cars will need to achieve 35.7 mpg and trucks will need to reach 28.6 mpg.

The rules were designed to push companies to boost fuel efficiency across their entire lineup. Manufacturers will have different requirements for cars and trucks of different sizes based on vehicle sales. Collectively, the fleet of new vehicles will need to meet the rules. Among individual manufacturers, passenger cars built in 2015 by General Motors will need to average 34.7 mpg, Ford’s cars will need to reach 35.5 mpg and Toyota’s cars will have to achieve 34.6 mpg.