A new paper from Resources for the Future summarizes the literature:

…gasoline taxes are a far more cost-effective policy than CAFE standards because they exploit more margins of behavior for reducing gasoline use. Austin and Dinan (2005) and Jacobsen (2010a) estimate that CAFE standards are about 2–3 times more costly than a gasoline tax for a given long-run reduction in fuel consumption. In Jacobsen’s (2010a) study, total welfare costs average about $2 per gallon of fuel saved for a 1 mpg increase in the CAFE standard, while a gasoline tax that saves the same amount of fuel imposes welfare costs of about $0.80 per gallon. The cost disadvantage of fuel economy standards is even more pronounced in the short  run, as fuel taxes give all motorists an immediate incentive to save fuel by driving less, while new vehicle standards only permeate the vehicle fleet gradually.

Yet despite their much higher cost, CAFE standards are more popular than gas taxes. Our desire to have costs hidden from us is a very expensive preference. The Obama administration was able to pass aggressive CAFE increases in 2009, in contrast both democrats and republicans were campaigning on a gas tax cut in the 2008 election. Would either party be receptive to abolishing CAFE standards in exchange for a higher gas tax? I doubt it, but it would be good for the environment and the economy.