The proposal, unveiled in June, would apply beginning in vehicle model year 2012 and gradually increase to an average fuel economy of 35.5 miles per gallon (15 kilometres per litre) by 2016, four years ahead of the current goal. Average fuel efficiency for the current model year is 25 miles per gallon (mpg).

The nation’s car and light-truck vehicle fleet accounts for about a fifth of total greenhouse gas emissions and represent about two-fifths of US oil consumption, according to the Environmental Defense Fund.