Long before 2020 the CAFE standard will become inconsequential as the price of oil soars much, much higher.
The recent discovery of a new oil field in Brazil is the largest find since 2000. At the present rate of world oil consumption, this new field represents a 100 day supply.
So, it took the world seven years to find a new 100 day supply. Now combine that with the very rapid growth of car sales in China and India and the future of oil is obvious; it's going to be much harder to get, and much more expensive to buy.
In five years or so, if oil is over $300 a barrel, and gasoline is about $10 a gallon in the US, electric cars will look very good to whole lot of people. There won't be much demand for big, thirsty vehicles anymore.