Dear Hsn18,
I dont believe what Xulfi has stated, since in Pakistan the fuel quality is so bad that it often chokes the catalytical convertors installed in imported cars/Local cars. But incidently if cataytical convertor is choke then, you car wont go above 40 ~60 km while pressing the pedal full down. Ironically in your post you haven't mentioned such a problem.
valve setting could not be the issue since its valve adjusts hydraullically.
Now you havent mentioned is your car is Type M, MX, XS or eco mode. since the economy of only M is stated as 17 km/Liter in Japan the rest provides 21~23.08 km/Litre in (10-15 mode) and in JC08 mode it is lesser. therefore it is suggested that:
You better try to drive you vehicle in both the modes and share the averages.
Try driving your vehicle in lower rpms.
Try using 40 % high octane and 60 % normal patrol.
Use good quality patrol (Caltax/Total/PARCO).
Use tyres of 155/65 R13 with 32 Psi in all tyres.
You havent stated in what speed you often drive the car?
Dont jerk up your car to get high speeds.
I myself own a 660 cc mazda carol where i get 17.34 km/Lit of averages in City with AC (10-15 mode).