Your hybrid Saturn is way more aerodynamic than most other cars out there so getting better fuel MPG return IS expected, and now with the TDI..., you should be seeing close to 70 per gallon!
What speeds are you computing your MPG's at?
Hills/mountains in this equation, windy?
Put a tail on that thing and flush the rear air exit, that is the ticket to get there...
, makes fuel last longer...