CAFE standards drive up the cost of cars. The high cost of cars means people keep thier old car for longer periods of time. Cars over time pollute more.
It used to be Americans bought a car around 5 years, now its 9 years.
In the 1980's the CAFE standars made smaller more fuel efficent rolling deathtraps. Directly resulting in thousands of deaths on the roads.
Hillary Clinton, Barack Obama and John McCain all are stumping for this "tax" that, like a typical government program is a road to hell paved with good intentions.