I installed GRECO sequential CNG kit which has two year warranty. Also It is very important that the kit should be fitted by professional. Many brands are available in market but fitting also plays an important role. Sequential CNG kit is costly but it’s one time investment & it increases the life of engine. Also some times one can drive the car on petrol also.
I installed the CNG kit in Pune around 45K with RTO passing. I drove the the car with CNG almost 10, 000 km & it worked fabulous. In city I got mileage approx. 25-29 km/kg & on highway 33-37km/kg(with AC).
Certainly this depends on your driving style also. But I calculated & got above mileage from my car.