Maybe we have the wrong sort of CAFE standards, according to this LA Times piece:

All told, livestock are responsible for 18% of greenhouse-gas emissions worldwide, according to the U.N. -- more than all the planes, trains and automobiles on the planet. And it's going to get a lot worse. As living standards rise in the developing world, so does its fondness for meat and dairy. Annual per-capita meat consumption in developing countries doubled from 31 pounds in 1980 to 62 pounds in 2002, according to the Food and Agriculture Organization, which expects global meat production to more than double by 2050. That means the environmental damage of ranching would have to be cut in half just to keep emissions at their current, dangerous level.

A University of Chicago study examined the average American diet and found that all the various energy inputs and livestock emissions involved in its production pump an extra 1.5 tons of CO2 into the air over the course of a year, which would be avoided by a vegetarian diet. Thus, the researchers found, cutting out meat would do more to reduce greenhouse gas emissions than trading in a gas guzzler for a hybrid car.