Liquid,
I absolutely agree with you that market-based mechanisms and effective competition create the best solutions in the fastest time, but want to point out that they do nothing for what are known in the economics world as externalities (the classic example of which is pollution.)
For example, if your 7-liter naturally-aspirated engine emitted 15 pounds of black soot for every mile you traveled, I don't care if you're willing to pay for the gas - you might be damaging my property and I'd have no way to get compensated for that! What I
can do is organize a group of people to get together, agree that NONE of us want our properties covered with smoke, and collectively make sure that none of our engines emit too much of it. That collective group (which we in the US call "government") is absolutely acting in a value-added role in this case.
I don't want to misinterpret your comment, so I'll make clear that I'm not answering whether or not CAFE standards are the best way to achieve overall fuel efficiency (since it's not as clearly obvious that CO2 is a pollutant like thick black smoke - a debate that's well beyond the scope of this forum.) I'm just saying that market-based mechanisms aren't always a perfect solution.
Giving people options and having them pay for what they use is an excellent idea - as long as they're paying for
all the costs incurred and not just the ones they see directly.
PS - Wikipedia has a great article on CAFE standards, which includes the excellent point that, although Europe has higher mileage standards than the US, several cars that meet their regulations wouldn't meet ours precisely because we have tighter regulations on non-controversial pollutants like soot.
http://en.wikipedia.org/wiki/Corporate_Average_Fuel_Economy