I stand correct, for normal drive >> O/D off indicator is lighted means the car overdrive function is OFF. 

Car owners nowadays don't read the user manual or even Google?

BTW... do you know roughly what other reason that may cause high FC for vios? I change gear oil edi. Service my car too.

Mechanic said mounting rosak but will not affect anything just that the car shake badly. tyre must change, nothing else liao... no mechanic could tell what happen.. i went 2 mechanics edi/...

BTW... do you know roughly what other reason that may cause high FC for vios? I change gear oil edi. Service my car too.

Mechanic said mounting rosak but will not affect anything just that the car shake badly. tyre must change, nothing else liao... no mechanic could tell what happen.. i went 2 mechanics edi/...

What is the condition of traffic?, if traffic is bad then FC is bad too. 

Is it you always wait in car and let the engine run?  This will use petrol with no milage for you to track. This case mostly happen for sales person waiting for appointment and parent wait for their kid to finish school. 

Tire need to always properly inflated

And also your driving habit

For the OD,  you already doing it right.  Just let it ON (light off)

the confusion in this thread is over 9000... lol

Rule of thumb: if there are any indicator lights (other than your headlights when at night) that remains consistently on when you are doing normal driving means you are doing something wrong or the car needs to be looked at.

its pretty simply really, any lights include O/D, ABS, Traction, 4x4, handbrake, battery sign, check engine should not be visible in normal use.

find a clear open highway......
1)test 1, od button clicked (light on)
-try drive at around 80 kmh (or whatever the speed limit is, faster = more noticeable)
-maintain said speed (try maintain the same speed aka needle no move, keep it at the same speed)
*to know what speed you are driving at, check your speedometer (the one with km/h)
-take note of the rpms (remember the figure)
*to know what the rpms are, check the tachometer, the meter with the rpm
-tabulate result
example: result: at 80 km/h, od light on, rpms are at around  3000rpm/3krpm

2)test 2, od button clicked again or 'disabled'(light off) (whatever you want to call it, select the other setting)
-try drive at around the same speed as before
-again, maintain said speed (try maintain the same speed aka needle no move, keep it at the same speed)
*to know what speed you are driving at, check the same meter, your speedometer (the one with km/h)
-again take note of the rpms (remember the figure again, do not confuse it with the other figure)
*to know what the rpms are, again check the tachometer, the meter with the rpm
-tabulate result
example: result: at 80 km/h, od light off, rpms are at around  2500rpm/2.5krpm

- the mode where you achieve lower rpms for the same speed... stick with that mode....
***examples are just examples u might not get those figures, just examples..... go forth and post your findings

edit: good luck   

i honestly cannot make it any easier than this 
eik whylah i type long version

Overdrive should stay on (light off) all the time for best fuel economy

Overdrive off (light on) means you are limiting the maximum gear to 3rd gear and it won't engage 4th gear (for old vios gearbox)

Of course you can do both for your "normal drive" vios gearbox is very hardy, you can literally throw it around and it still work

Car owners nowadays don't read the user manual or even Google?

I did on engine every weekdays for roughly an hour, without car moving.. Cause waiting waiting... So that's one of the factor that caused fc high too? Hmm...  Then let me try not turning on engine while waiting and see it improves the fc

Only Vios owners.
They know nuts about cars and just buy em for the badge.

Thanks both sifu!! It does explained and I'll try it too hehe..

I did on engine every weekdays for roughly an hour, without car moving.. Cause waiting waiting... So that's one of the factor that caused fc high too? Hmm...  Then let me try not turning on engine while waiting and see it improves the 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