Whenever my Prius mileage drops down that low it is because of weather. And I'm not even a hypermiler. Fortunately, I'm not paying the diesel price premium either...
The only way to up the fuel economy that much is to disable the emissions control. When the EU mandated SCR/DPF on new diesel engines, fuel economy plummeted The city of London was faced with air quality problems due to the use of diesel engines. In response, they mandated retrofitting SCR/DPF's to all commercial vehicles and buses Emissions standards | Transport for London http://www.london.gov.uk/mayor/environment/air_quality/docs/fleet_op/reduction.pdf http://www.topsoe.com/research/Reso...ers/Topsoe_new_emissions_cont_syst_trial.ashx Up until last year, I used to frequently travel to the UK, France, Germany, Switzerland, Singapore, and China, on business. Overall, the air quality in North American cities is much better than in the EU
There is no such thing as a North American 2008 Jetta TDI. Do you mean a 2009, or did you import your car from Europe? What exactly does your ecm reprogram do, and who did it? Does it disable the DPF and NOx trap? That is an easy way to gain some mpg, but nowhere near enough to push you to near 70. I've never heard of a reprogram giving that much of a mileage boost, and I do read around at tdiclub on ecm reprograms every once in a while. Good luck with the WVO. I'd be amazed if you made it 50k miles on WVO without needing some serious work. Even B100 is pushing it in the 2.0 TDI.
Aside from that fact that fueleconony.gov indicates that the 2007 and 2008 Jetta TDIs don't exist --- 48 mpg from a 2009 TDI appears to be a very good start, but she still has room for improvement. Remember that Aussie couple that drove our lower-48 states last year, getting 58 mpg? This seems to be the reference point on VW TDI Tank Wars. This spring, a couple friendly contributors here -- including one already in this thread -- helped push this same model over 66 mpg, over 1920 miles. This is my first year with a high-mpg vehicle, so I'm still learning. With the increasing cold, wind, and rain, my mpg just dropped under 50 for the first time, and is likely to stay under until spring weather returns.