I have kept a Khyber for almost 10 years and an alto for 4 years and I find the Khyber to be far more economical on petrol than the alto is,
I haven't used a Khyber on CNG but use my Alto on CNG and it gives me a full tank on good pressure for around 510 to 530 rupees which gives me a maximum of 100km with a tuned engine and new spark plugs etc. On petrol however the Alto is rubbish, I have also kept the older shape / pre-facelift alto for a couple of years and that too is rubbish on petrol, it is actually more economical to drive the 2010 Corolla or City on petrol than it is to drive the alto on petrol.