I'm getting 18-20 mpg (u.s. gal) average with mostly city driving and some county road driving. Considering it's winter, & I'm doing probably 70% city on that figure, I'm pretty happy with that. Before it turned cold, a hunting trip 3 + hours away I scored just shy of 29 mpg round trip hand calculated. Just a couple weeks ago I drove 2.5 hours away and round trip was right around 26 mpg.
To answer your question though, there are several threads on here that will probably directly give you some answers. Feel free to search, but I will throw out some basic information for you. Things such as winter blend diesel, cold temps, drive line fluids being thicker when it's cold, snow on the roads equates more rolling resistance, warm up time is fuel being burnt without miles driven which equates to bad mileage, etc.