@Poster, Ive owned Santro Exec 2004, had it on Cng, car is excellent especially comfort, acceleration, a/c and other stuff, BUT Exec doesnt comes with f/f cng so you'll have to install cng AND THAT WILL MAKE YOUR LIFE MISERABLE BELIEVE ME, This is my personal experience about Exec, go for it if you opt to run it on petrol otherwise for cng go buy club! (y) drive of exec is much better like power steering, power windows etc but CNG made my life hell. Anyways thats my personal experience and this might have happened with me only so asks some experts aswell before making final decision and best of luck!.
Respect and care about your car rather than abusing it on the streets.