Most articles focused mainly on the vastly improved 'corporate average fuel economy' (CAFE) standards of the ruling. The Union of Concerned Scientists provides the historic background on the involvement of EPA.
"The White House finalized (on April 1) new clean car rules from the Environmental Protection Agency (EPA) and Department of Transportation, securing the largest boost in fuel economy in decades and, for the first time, using the Clean Air Act to require reductions in the amount of heat-trapping emissions from cars and light trucks.
The joint rule will boost the average fleet-wide fuel economy of new vehicles sold in the United States to 34.1 miles per gallon by model year 2016. The standards also set national global warming pollution standards for vehicles at 250 grams per mile, roughly 25 percent less than the emissions produced by today's average new vehicle.
In 2007 the U.S. Supreme Court's decision in Massachusetts v. EPA affirmed that the Clean Air Act gives authority to EPA and California to regulate greenhouse gas emissions. This ruling also directed EPA to address any threat climate change poses to human health and welfare. This legal decision formed the foundation for the EPA standards finalized today."
Note: See bottom of pg. 2. of UCS fact sheet to understand why they listed 34.1 mpg as the 2016 fleet average while most media outlets listed 35.5 mpg.