I get around 16lts around town with a very heavy foot. I have always used 98 octane fuel since I bought the car around 17k ago. Its done around 23k and I get the oil and filter changed every six months and use full syntehtic oil. I also have a cat back exhaust which helped me improve economy, it was worst before the exhaust. I have noticed one thing which was bought up in some previous thread that after a few weeks of feeding premium fuel fill the tank with the 91 octane, has helped me with the mileage, try doing this.