Presumably it’s to save on oil:
Yet fuel efficient cars effectively means cheaper energy which in turn means they will be used more – instead of, for example, using public transport.

Fuel efficiency is of course an advantage people can consider when buying a car – and can compare with advantages that inefficient cars can have (speed or greater safety because of greater weight, etc, as well as a probably lower price – or they would be efficient already).

As far as governemrnt is concerned, any oil shortage – for geopolitical or economic demand reasons – raises the gasolene price and – guess what – increases demand for fuel-efficient cars anyway, no need to legislate for it.

A final reason is that – as research at Georgia Tech has shown – it is possible to clean emissions of CO2 (and other substances at the same time), and that therefore
a fuel neutral emission-tax or similar makes more sense:
If it is economical to make gas-guzzling cars with emission processing, then, again, there is no reason for government to prevent it.

Any regulatory measures should therefore focus on emissions, rather than the fuel used, and emission taxation retains consumer choice, while still giving government income with the lower sales of such cars.
(Regardless of whether CO2 reduction makes any sense, lowered emissions of course have their own benefit, for all the noxious sulphur etc substances that the emissions also contain)

http://www.ceolas.net/#cc25x