That will be challenging! Even the VW Lupo 3L (
Volkswagen Lupo - Wikipedia, the free encyclopedia), which was designed to get 3L/100km (78mpg) on the Euro equivalent of the EPA ratings with a teeny 1.2L 3cyl turbodiesel, has a hard time getting to 75mpg in real world driving (see the ecomodder diesel fleet FE rankings:
EcoModder Fleet list - EcoModder.com). Also notice that there are no diesels newer than 2010 in the top 20...
Also, check out Fuelly to get another perspective:
Volkswagen Beetle MPG Reports | Fuelly
That said, it doesn't mean it's impossible... but probably extremely difficult. If you're still looking for a TDI, consider getting an older TDI with an ALH engine, available in the 1999.5 to 2003 MkIV VWs. With mine, I can regularly get high 50's, low 60's per tank. There are a few others on Fuelly:
2003 Volkswagen Jetta MPG Reports | Fuelly