Think of it in a physics way. If you are using something to take away energy (lowering temperature), u need energy to take it out. Where do u get your energy? by burning fuel lor.
So the lower the temp thermostat u set, the more energy that it has to take out. So more fuel burnt for your ride's comfort.
Turning the fan up just uses the motor power. Takes a little bit of energy compared to using more aircon strength to bring down the temperature.
The reason why people wind down their windows during long journeys is not because they want to save petrol. Is because with a/c on for long journey your engine will overheat. So they wind down windows. This info is told by my dad. I have no experience on it. So pardon me if I said anything wrong.
As for what I do, on a hot sunny day when i enter my car, i'll set it all the way to 18 degrees and turn the fan to max for abt 1min. Then after that i will set it back to 25deg and speed 2. until i shiok liao then turn to speed 1. As to why i do it, i dunno also.... habit liao. I just want to have a comfortable ride. If setting at 18deg will make me comfortable, i'll stick to it. no point bothering so much on FC savings when it will change your driving habit. :)
Cuz if i really want to bother abt FC, i'll get a van - Good FC @ 20km/l, a/c set to max also can... 2000cc pick up faster than latio on full load somemore!! rev hard hard, diesel cheap!
