Hey there all! I'm new here... Just wanted to share some experience!
Got a '99.5 Jetta TDI (mk4) with 310000 km (193000 miles). I bought it last November to replace my previous '97 turbo diesel (non-TDI) Jetta, which was totalled in a crash at 475000 km (295000 miles!) but was still pulling strong!
So far my average MPG on the new Jetta isn't that great, because of our cold canadian Winter and the according Winter fuel... and I don't always drive very efficiently. Well, I sometimes keep it at 120km/h (75mph) on highways in good weather, but that's my only bad behavior. I'm quite careful otherwise. I coast before stopping, I plan my trips, etc. without hypermiling.
Average after 7 month and 13000km (8000 miles) is a little bit under 6L/100km (40 MPG (US)), with a best of 5.46L/100km (43 MPG). I'm expecting better results from now on with a new upgraded ECU I installed last weekend. If I can resist the temptation of the now unleashed 210 ft-lb of torque.
So, I'm looking to reach 4.7L/100km (50MPG) at least once this summer... Good weather, longer trips, vacations with no hurries (cruise control set at speed limits) sure will help.