you will notice when you driving and turn it off (light on), car will drop gear.

OD off (lights on) only when you want to overtake / naik bukit with low gears. if normal chill santai drive, just let it on (light off)

you did it right, just turn OD on (light off) during normal driving. but your FC is scary tho. My dad's Vios year 07 spec G, full tank can go ~380km. better ask mechanic check other things.

Your car problem since you bought 10 years ago or just happened recently?? if since you bought it, might be your car system was wrongly set up, supposedly OD on (light off), FC cantek. OD off (light on), FC high. But based on your FC experience, your car's set up OD on (light on), FC high... not sure you understand or not

So best way is, get your dad / brother / friend or whoever you think can help you to test your car. No need good pomen to recognize this OD functionality.

I'm no expert, but check these 2 basic first. If all good then it might be relating to the internal parts of your engine.

EO change on time? if use semi syn, around 5-6k odo must change. if use fully syn up to 10k odo then change. If no change on time, dalam engine sudah berat because it's not lubricating the moving parts good enough.

changed to bigger sport rim? bigger s/rim makes your car heavier

driving style? city drive everyday jam also contributes high FC

wahlao engine run 1 hour without moving weh... for sure FC damn high one. try to avoid that habit, when you stop more than 5 minutes, just roll down windows and off the engine