brother i doubt on ur calculations, dont take on heart
see yourself from where u get reading infact car average will be same as if on any fuel if u r getting 13 km per litre, similar u will get 13 km/kg
fuel type will not disturb your mielage so i am rewriting your correct miealge as:
Petrol 1 Liter= 105.68 Covers 13 Km/l.
CNG 1 KG =89.10 Covers 13 Km/Kg
Difference in Rupees is 16 per km
So i think CNG is yet feasible,if you daily drive average 100 km, this means daily u can save 1600 Rs easily & in month u can save 1600 * 30= 48000
so within a month u will get your cng investment back
also your car will be good with both type of fuel