Coming to the technicalities of your measure, very few cars have double the fuel efficiency of the fleet and very few have half. So you'd want more tiers in the schedule. Something like 8 should do.

Taking CO2 emissions, the Porsche Cayenne entry model, for instance, has 310 grammes of CO2 emissions per kilometre, which is less than double the emissions of the EU average (about 160). There are no new cars that emit 80 grammes. The most efficient new car now, the Toyota Aygo/Peugot 107/Citroen C1 model, emits 109 grammes.

80 should be possible if you make those hybrids; the Honda Insight emitted 80 grammes according to Honda (probably a bit optimistic, but it was a larger car).