I have a 2006 Beetle TDI BEW PD engine with 6 speed DSG tranny and consistantly get 37-40 back and forth to work. Last year, on a four hour highway trip, I got 43 mpg. I just completed a 100,000 service (changed timing belt) at 74,000 so I'll keep an eye on fuel mileage. I don't expect much change as the engine ran well prior to the service.
All items listed can surely lower your mileage but 30 is really low. Do you have a fuel leak or is your exhaust blowing black smoke?