As expected, the real-world fuel economy of the Civic diesel is lower than what Honda claims. This is due to the fact that the claimed fuel economy figures stated by the carmakers are recorded in a controlled environment.
If your regular commute is in city conditions, then expect an average mileage of around 17kmpl. However, it could drop further in dense traffic conditions.
But if your travel involves more highway stretches or a relatively clear route, the overall fuel efficiency should go up by about 2kmpl.