Hi, how long have you had the car? I'm just wondering if you had it last winter, or if you're comparing it to mileage you got in the summer/fall. What I've been told is that there is a big difference in fuel economy between summer and winter with diesels. Maybe even as much as 20%. The engine has to work that much harder, plus we tend to let them 'warm up' more before we even start driving. That said, there may still be ways to improve. There are some hints on the website
www.fueleconomy.gov.
Good luck!
Natalie