Sir,
I am a CNG user and drive an eco fuel car.Now, you would appreciate that when it is a petrol car and you intend to fill the fuel, you normally use currency conventions, as in filling 500/= or 1000/= fuel an so there is no direct loss of consumer.
In the case of CNG, generally the consumers go for filling upto the capacity as it saves time and also the cost of filling a 12 Kg clylinder is never above 225...