In City
My car does 100 km in 7.6 kg of CNG
and it does 100 km in 9 Litrs of Petrol
i drive about 500 km in complete month
So i save 9*108 - 7.6*75 = 402 Rs for every 100 km if i use 100% CNG
so in a month i could save 402*5 = Rs 2010/- Per Months
It is not possible to run your EFI car on 100% CNG, i.e No petrol At all
I assume that 5% of fuel is consumed for (Starting and Initial Idling) in any case = 2010*0.95 = Saving 1909/- Rs per Month
But actually in Current CNG outages maximum i managed to get 55% cng and 45% petrol (it is even worse nowdays)
So Savings = 1909 * 0.45 /0.55 = About Rs 1500 Per Month
Now I also waste Rs about 10 km for every CNG Refill from nearest station, which means (if 10km) 5 km of petrol wasted and 5 km of cng wasted = about 60 Rs of Petrol and 40 Rs of CNG and i do about 400 Rs per month WASTE )
So Savings = 1500 - 400 = 1100 Rs Per Month
to mention the idling waste at CNG station waiting in long lines)
about 40 minutes to 1.5 hour i think about 50 rs of fuel is wasted per fill
So Savings = 1100 - 4*50 = 900 Rs Per Month
Now if i am cranking my ignition at every 5 minutes for about 1 hr or so my ignition system is worn out for about 12 turns extra , this wear tear will cause its life reduced, Your Cylinders life also reduces while working on CNG. Lets keep Rs 500 to cover this cost.
So Savings = 900 -500 = 400 Rs Per Month
Now if i have only 4 fills possible it wasted my precious time of about 3-4 hours per month. If i pay any person to go to cng station and fill my car for about 1 hour he would take at least at least RS 100 for it per hour
So Savings = 400 - 400 = ZERO:o
So i decided to Forget about this CNG stuff and maybe in future i may be able to get a fuel efficient car and also i would try to reduce my driving a bit and Chill