Interesting that you call 12-18 km per litre fuel efficient. That's 8.3L-5.6L/100km.
My Passat uses a steady 6.5L/100km with a 1.9L engine and I don't call that very efficient. 1.4L engines should go qith 4-5L/100km in my opinion. 
You can do that in a relatively friendly traffic. During the Ramadhan, when everybody left Jakarta for their hometown, the traffic was sparse, I once did a 5L/100km.
A VW Golf (that same 1.9L engine) here will do 11L/100km and be considered great already. I don't know if it was for:
- poor fuel quality (I use Shell anyway, our local gas station are even worse, but there are alternatives like Total and Petronas)
- poor air quality, air filter quickly gets dirty here, a Swedish mechanic friend of my father advised to clean it every month and change it every year.
One of my friend drives a Scirocco (turbocharged 2 litre), it gives him 5-7 km per litre. His close friend's car, "economical" Honda Civic with 2 litre (K20, with non performance camshaft) gives him 6-8 km per litre.
But one thing that cause the inefficiency for certain: traffic conditions!
- traffic jams are the worst part, if it wasn't for public transport stopping/parking all over the place, people parked their cars off-street as they wish... both making crowded street even more cramped.
- traffic lights that use timer instead of sensor. And they are not even timed properly, causing long queue in one direction, and empty street in the other.
- traffic managements are even worse. In Singapore and other civilised countries, people queue behind traffic light line when the traffic across the street is too full for you to squeeze in, otherwise you might block incoming traffic from other direction. No, people here just squeeze in and block the other traffic flow instead, because those queuing behind you are impatient either.
- Driving behavior. It is insanely easy to get driver license here, so people are sometimes drive stupidly. e.g. taking too long to move when the light is green, wasting several precious seconds. One of fuel-wasting behavior is to stupidly keeping huge distance when the traffic is stuck, prompting us to brake too often, just because the guy at the front is not confident. Sometimes they brake too prematurely, requiring both of you to hit the gas pedal again to close the gap with the front traffic.
- I guess the stereotype "Asians can't drive" is somewhat correct to certain extent here. People here do not adhere to lane lines, some are not confident with the size of their cars, so they tend to maintain space in their side of driving seat, making them leaning to the left (drivers sit on the right hand side here). The problem is, people who want to overtake lanes rarely signal with lights, they just lean to the lanes they are going to take, until they completely intimidate others driving on that lane, then they just enter it. It makes you brake often, and it wastes fuel.
- Impatient asshole drivers, so you are driving straight in a relatively empty lane, so you have to maintain separation (due to speed). But then some asshole from the other lane take over you, because he/she suddenly doesn't want to queue on that lane forcing you to hit brake.
tip: if you drive here, flashing lights mean you have to get out of his/her way. In Japan and other civilised places, it means they are giving way to you, however it means the otherwise here. It is an addition to your horn, so you have both audio and visual tool to make your way through other drivers here.
If you say 6 hours, it isn't 6 hours for the distance, but 6 hours for the traffic jams, well mostly.
To get driving license here:
You just need to apply for one, go to the traffic management section of central police station, take the 1-day full test (a lot of bribery), then get the license already. I learned that it was unlike in Europe, US, Canada, or Australia... where you have to pass through course stages for several months. In NSW, Australia, they label your cars by putting a sticker in your license plate if you are still noobs/on probation. During early stage of application, you must be accompanied by a fully licensed driver and can't drive unsupervised.
I learned how to drive properly there, after 4 years of getting Indonesian driving license and drive recklessly like most of my fellows. But Aussies in NSW have this funny policy of allowing us to use our license there (for 3 months, then you need to "translate" it to be fully legal for the rest of the time), so I don't need to reapply and relearn, just read more road signs then what I usually did in Jakarta and imitate people's courtesy behavior on streeets (e.g. if you are given chance to take his/her lane, you wave your hand on your mirror to say 'thanks'), and that's all. It is fun, esp. when not in downtown city, where impatient South Asian taxi drivers crawled all over the streets. My instructor back there is a German, btw and the car is owned by a Chinese guy, both are Australian native.
