Cars Q&A

I bought a Lancer GLXD recently. It is a used car around 9 years old. I was told that it would fetch around 18 kms mileage on diesel. But in my experience it gives only around 12 kms. What is the actual mileage of a diesel lancer, both city and highway. What is to be done to increase mileage. What is the driving speed for maximum mileage. Please help.