I was also getting 8-9 kmpl on my 7500km driven swift keeping rpm max to 3k rpm, stock plugs. I used to redline occassionally too when in the mood
Now im getting 11-12 easy.
What did i do?
I started ALWAYS keeping revs absolutely below 2000rpm , and the occassional hard drive the same.
Changing shift from 3000 to 2000 rpm made a HUGE difference. Also, i changed to caltex and pso, never use shell, maybe the shell petrol pump on my route is bad. And get plugs checked, ignition timing set.
Conclusively, shift at 2k, use caltex. Occassional hard drive is fine. My personal and honest opinion however stupid or insignificant it ay sound, these newer small engines consume a lot when driven hard.