Car purchases, especially a new car, are rather big investments. I think people do really think about the future costs. One can enforce, that every seller of a car has to declare the amount of gas a car needs per 100km or something like that, so that people really know what they are buying, but I think this anyhow already now the case, too.
Then what is a rational decision? Is it a rational decision to go to the cinema on saturday evening? Sure. And in the same way it is a rational decision to buy a car, which has more power than neccessary. When the society decides we can emit the amount X of CO2 next year, then it is the usual way to let people buy shares of this emission rights from the state (that is equicalent to fuel tax, which is readjusted somehow to match the overall target). It is market economy to assume that those who are willing to pay the highest price, are those who will have the most usage.
 

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers