I absolutely agree with Jinn, my cars run on CNG and there are two things to notice here...
Diesel cars give more mileage, even the Corolla when compared to a petrol variant like 600 KMS [D] to 400 KMS [P]
Mixture has to be adjusted every few days + cars have low power feel whereas Diesel cars feel immensely powerful when compared to CNG
My VTI Prosmatec gives like 80 KM to a Rs. 225 tank full over CNG, it's low but then again it would be going 60 KMS in a Rs. 500 tank full over petrol. Diesel still seems the best option out there.