The cornerstone of Democratic presidential hopeful Hillary Clinton's plan for energy and climate change is a call for 55 mpg fuel efficiency by 2030.

The goal, far more ambitious than current levels being debated by Congress, would help to achieve an 80 percent reduction in greenhouse gas emissions by 2050 from levels measured in 1990.

At the same time, the Clinton plan would cut American oil imports by two-thirds, some 10 million barrels a day by 2030. More than half of the daily consumption of oil in the United States is attributed to gasoline usage.

Under the plan manufacturers would benefit from a bond program totally $20 billion to finance factory refits and conversion to alternate fuel technologies.

The national goal for renewable fuel currently set at a target of 7.5 billion gallons per year by 2012 would be extended to 36 billion gallons in 2022 and 60 billion in 2030.

"This is the biggest challenge we've faced in a generation, a challenge to our economy, our security, our health, and our planet," said Clinton in a statement issued by her campaign outlining the details of the proposed plan.

Predictably, automakers are characterizing the goals as unachievable with the most criticism coming from the Detroit Big Three, General Motors, Ford Motor Co., and Chrysler LLC.