i have exi prosmat 2004 with 55 cylnd. and i have done 45000km in 15 month only in city without any trouble. its give me 120km with AC , and 140km in without ac. i didnt try in on motorway.
its all depend where from u get the cng and wht time. and its also depand ur moving around in the city.
remmber always fill cng from one station and in night time if posible. and try to fill cng from private cng stations.