The EU commission truly lost the plot, of the emissions produced by a car, diesel or petrol, the only exhaust gas constituent not subject to defined not-to-be-exceed limits is the CO2 content, all other constituents, Nox, PMs etc they had defined do not exceed number, ie Nox 180 unit/km , or 80 units/km, for CO2, you could throw out as much as you wish provided the car maker accepted the one off penalty charge and the buyer paid the VED rate.
CO2 is simply fuel consumed by another name. the "regulated" pollutants are the toxic ones, and a manageable unfortunate by-product of the vagaries of combustion of HC fuels in air (N2 and O2). By manageable I mean they can be modified to a greater or lesser extent by engineering the combustion system, pressures and temperatures, and after-treatments etc.
CO2 and H2O are just a consequence of burning HC fuel, the more you burn the more CO2 it produces, unavoidably in principle.
Fuel consumption is principally a function of vehicle mass and size (area, drag). At one time there was a table of "ton.miles/gallon" for many production cars, today's equivalent comes out at somewhere round the 50 tonne.miles/gallon (imp) mark, in other words a 1000kg car will do around 50mpg (imp) typically over a mixed drive cycle, maybe a little better.
Physics and thermodynamics say that with a "normal" ICE burning liquid HC you can't do vastly better than this. The principal way to lower the CO2 values is to make cars smaller and lighter with lower performance levels (operating the engine at nearer optimum efficiency regions). That isn't the trend for a lot of cars, they get bigger (ergonomics for modern people), heavier (more gizmos and better crash performance), and higher performance (desirability/sales&marketing).
The USA/California have had CO2/gas guzzler taxes and penalties for decades, which were paid by the manufacturer. If you impose a CO2 limit you essentially limit the size/weight of an ICE vehicle, which is a fundamental step change in regulatory principle. Some markets in effect have such rules for certain segments (Japan and kei-cars).
EV/hybrid sort of by-passes that issue by putting the CO2 somewhere else (power stations).