Going back to your original question, there is a lot to be considered with diesel cars, from 2009 in Europe at least:
The DPF introduced then is not like the Catalyst in a petrol(gas) engine - or indeed the pre-cat that comes before the DPF proper - in that it is not a full-flow device.......picture looking down a bundle of straight tubes: you can see out the other ends (like looking in one of my ears & seeing who's stood the other side of me
)...this is like a Catalyst.
If you were to look in the end of your DPF you would see that half your tubes are blanked-off, and the other half that are open you cannot see out the other ends. The exhaust gas has to be pushed THROUGH the walls of the tubes to the adjacent tube which is open at the exit.
This odd state of affairs is necessary to TRAP the diesel soot particles (the black smoke that gives you lung problems, smog, etc) and it does this very well.
The huge fly in this environmental ointment is that from first starting your diesel car (0 miles/kilometres) you start blocking the DPF and increasing the back pressure on your precious engine. Certainly by around 45 percent blocked you will have a drop in performance, an increase in fuel consumption and the engine management unit (reading differential pressure across the unit) will have instigated a "regeneration".
In most cars this involves (sometimes as often as every 2-300 miles) adding an injection sequence (squirting neat diesel) into the cylinders on the exhaust stroke. This fuel burns in the exhaust and raises the temperature to perhaps 500 degs centigrade, so burning the accumulated soot. This may result in some white ash being puffed-out from the tailpipe and it will certainly make the exhaust (including tailpipe) VERY hot. This will last up to 20 minutes (variable) and adds a fair bit to your fuel consumption. NOW....
IF you do longish, regular "A" road/motorway runs (at least every week I would suggest),
IF you do NOT interrupt the Regen (by slowing down/stopping/being in neutral),
IF your driving isn't mainly around town/start-stop,
then you may find your DPF will provide fairly long service......
but most UK drivers struggle to do this and end-up in engine management hell (culminating in "limp-home mode"/loss of hair/loss of will to live
)
So, in summary: (In Europe, at least)
You will never get to match those test figures (even if you don't own a VW
) - mine suggests a 48 mpg average. Try as I may I cannot average better than 43-44, rising to 48ish on a long run with trip reset.
My X-drive 2.0D is obvious a bit worse than the S-Drive that some here possess.
As a thought to illustrate my ramblings:
My last car was a Sept 2006 E91 330D.....
At 108k miles it was struggling to regenerate (average consumption 35.8 mpg rising to 44-45 on a run)
I had the DPF removed from the box (the pre-Cat isn't really an issue, as mentioned above, and the exhaust smells without it) and the box went back in with all leads and pressure pick-ups replaced as it has to be there under EU regulations. The EMU had to be re-programmed to delete the regen cycle or else every 300 miles you would produce a smoke screen worthy of a fleeing Great War battle fleet
.
Over the next 10k miles I regularly checked consumption - a fairly steady 44 mpg average and 53-55 mpg on a run (trip reset).
The cars' new owner loves the car (and I miss it!)