If the consumer wanted high MPG vehicles, they would buy them. There would be no need for such a regulation if that's what people wanted.
Regulation is sought because "we the people" in our allegedly free nation are not behaving as our representatives (who think they should lead us) think we should.
Such regulation has no place in a self governed, freedom loving nation.
If you want a high MPG car, buy one. But don't force others to buy what YOU think they should buy. Make a compelling case that will convince them, rather than trying to force behavior through regulation.
I don't care if it's left wing authoritarian actions or right wing authoritarian actions. In a free nation, authoritarian actions are antithetical to freedom.