Let us assume that we keep the same cars than we have today in terms of weight, equipment and speed. Now, they emit, for the most recent ones, 140 g/km. The current yield (if this is the term) of thermic motors is about 30%. Having a 70g/km emission means to reach a 67% yield of thermic motors, which has never been done before (specialists consider that 30% is the maximum for now and maybe for a long long time).

This is not very realistic. And anyway, this means that with the cars we have, a 47 g/km emission is a minimum given by the laws of physics.