If I can afford petrol, I won't install CNG.
I was in search of a car for the last few months, and I observed the cars with CNG installed and the ones with CNG never installed had a very noticeable difference in engine condition and sound. Now I have seen more than 100 cars so this can't be just a coincidence. The ones driven throughout on petrol were always better, noticeably.
I love saving money, but this is one thing I will not suggest saving money from.