There are deeper reasons why the scheme is wrong.

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 government 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.

Another 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). A fuel-neutral emission tax on cars therefore 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 on cars retains consumer choice, while also giving significant government income with the lower sales of such cars, income that can go to projects that themselves lower emissions eg electric car manufacturing subsidies etc.
(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)

For more see http://www.ceolas.net/#cc25x