Mileage standards are a good thing
If anything they are too lenient. Mileage standards are a good idea, an analogy being emission standards which worked so well in the 70s to cut air pollution.
It's the government's job in a free market economy to correct negative externalities from industry. Global warming and dependence on hostile nations for energy are two major negative externalities. Legislation to change these unfavorable outcomes falls directly under the government's purview in any neoclassical economic model.