Aerodynamically speaking

[Gerrycan]

Most manufacturers achieve these high CD values with many "tricks". The wipers are removed; the external mirrors are removed; the road wheels (and ventilated brakes) are stationary; the radiator intake is blanked off; brake ducts are blanked off; any external antennae are removed; etc. Now they do not remove all, but they do all do something to get the best value.

In the real world, you have turbulent air, and the direction is not always the best. You have a whole lot of other factors that come into it.

All a CD value does is give you SOME idea as to its efficiency, however unless you know how it was achieved, it does not tell you much. It's a lot like fuel economy figures, and ANCAP ratings. They do not always tell the whole story.

You are right, but in some markets like the UK the claimed fuel consumption is very important as it determines the road-tax paid on the car. Even here the luxury car tax is affected by the claimed fuel consumption so I think they do look at all factors including aerodynamics.

When you look around the Octavia 3 there are a lot of little tweaks to improve aero.
Underneath there are small tab deflectors in front of the wheels front and back, the bottom is relatively smooth until you get to the large gap under the rear seat. The 'cavity' (200 litres?) is big and empty on mine because I have the torsion bar suspension but I assume it is provision for multi-link rear, 4WD, (or even batteries for future hybrids?). In front of the cavity is a plastic ramp with 'turbulance' fins to carry the air beyond the gap to the rear wheel well and further airflow smoothing plastic.
The engine air cooling (which accounts for 35% of drag) has been modified since my Octavia 2, on that car the radiators just sat at the front of the engine. Now on the Octavia 3 the air is ducted through the front vent to the radiator and has to pass through it, it cannot go around it. Behind the radiator there is one fan to suck air through when required which covers about 2/3 the surface area, the rest is covered by one-way flappy plastic pieces so that when moving forward they open naturally but when the car is stationary and the fan going then hot air has to vent underneath not back through what would be uncovered radiator.
On the body there are subtle curves and creases at the rear to cut off the attached airflow and reduce turbulence. There are better and more effective ways of doing this but they usually compromise looks or internal packaging.
Overall it is not a bad job and the manufacturers would not include all this stuff if it did not work, because most of it is not obvious.