nowadays all cars are auto-transmission, means the car will automatically change the gear for you depending on the speed you are driving.

1. car starting to move (i.e. 0km/h - 15km/h), need more full push power, so the car will automatically choose low gear (gear-1)
2. when car moving slowly (i.e. 16km/h - 30km/h), the car moving got a bit of momentum dy but momentum just a bit, so still need strong push power but less than full pull power, the car will automatically change from gear-1 to gear-2
3. when car moving a bit faster (i.e. 31km/h - 50km/h), the car moving with some momentum dy, so need less push power, the car will automatically change from gear-2 to gear-3
4. when car moving 50km/h and above, the speed of the car with momentum is also helping to push the car, so need least push power, the car will automatically change from gear-3 to gear-4
5. when you slowdown due to traffic ahead (i.e 31km/h - 50km/h), momentum reduced, need again more push power, car automatically change your gear from gear-4 to gear-3

So the gear number is depending on how fast you are going and how much push power you need, if wrong gear number vs. needed push power, the car will not have enough power to move, this will only happens on manual-transmission gear car, like using gear-3 to start moving a car (0km/h - 15km/h), the car engine will die immediately.

so auto-trasmission car (not those CVT type), normally will have 4 gears (gear-1, gear-2, gear-3 and gear-4), all this 4 gear will be used for your normal driving, the car will automatically change the suitable gear-number for you depending on your speed then.

OD means over-drive, turn it on is to reduce the number gears the car can use, i.e. change from 4-gears to 3-gears (gear-1, gear-2 and gear-3 only), this is to give the car some extra power, i.e. going uphill like Genting, or because you need some extra boost to overtake car in front.

Your FC is high because you turn on the OD, limiting the gear to gear-3 only, but your normal driving I guess should be faster than 40-50km/h, maybe you will be driving at 60-70km/h, so the engine will be very stress, working on small gear but at high speed with high RPM, hence consume more petrol.

Note:
Proper way to over take a car in front: lets say you are driving on 2nd fast lane at 60km/h (gear-4), the car in front of you is driving also driving 60km/h, but then you decided to over take him, look at right side no car coming, turn on over-drive (car will change from gear-4 to gear-3), maneuver the steering wheel to the faster lane, press the accelerator, it will give the car a bit of extra boosts because of gear-3, RPM/torque will change to higher/extra push power, engine sounds will be louder, when your car is at same level with the car you want to overtake, turnoff the overdrive, so the car will change from gear-3 back back to gear-4 and not to stress the engine so much and also give you back your full speed.

Overdrive Layman Term : a short time turbo button for speeding, or long time extra power to climb uphill.

Hope this help.