UPDATE 1-US plan would force gradual changes on automakers

* Proposal would cut carbon emissions by 21 pct

* Proposal would cost automakers estimated $60 bln

* Vehicle prices would rise under proposal (Adds details of rule proposal)

WASHINGTON, Sept 15 - U.S. automakers would be required to gradually increase fuel efficiency under a government proposal unveiled on Tuesday to raise the industry's fleetwide average to 35.5 miles per gallon by 2016 and regulate tailpipe emissions for the first time.

The 40 percent mileage increase over today's standard for passenger cars and light trucks, including pickups and sport utilities, is expected to cost the industry roughly $60 billion to achieve, according to the proposal unveiled by the Transportation Department and the Environmental Protection Agency (EPA).

The proposal would cut carbon emissions by 21 percent by 2030 and save 1.8 billion barrels of oil. It would boost passenger car fuel economy from 33.6 mpg in 2012 to 38 mpg in 2016. Light truck efficiency would rise from 25 mpg in 2012 to 28.3 mpg by 2016.

The proposal would increase average new vehicle prices from $476 per vehicle in model year 2012 to $1,091 in 2016, the government said.