bro im no tech at this but there are a lot of factors..long list 
lets c IMO
-well tuned car (cng and petrol) (clean filter,plugs,wires..and should not knock when u press pedal suddenly at low speed)
-get throttle body serviced..(caution:qualified person..also may or may not effect mileage 
mine didnt effect, in ur case may work as its supposed to be clean..)
-ist of all around 200 in city!!..the terms varies..my daily drive involves highway ie 5th gear max usage.. (200 means no traffic jams included)
-condition of car matters (mine was untouched engine wise also spot on although 125k odo)
-CNG quality matters,stick to one good cng station,in isloo cng quality is v gud so that may b a v valid cause of < mileage
-driving style matters (don't stretch it.. try these gear shifts (1st(0-10),2nd(10-20) so on such dat @60-65kms turn to fifth) no need to drag in low gears when u dont need to..
-turn to neutral on signals and long stops,turn lights off at night when u dont need 'em
-dont do unnecessary revving of engine, long engine warm ups after starts
they are small things but do wonders in long run and add up to increase mileage
hope its of use to u
peace and AH
