My friend's mehran does 100km on full Cng tank without AC (i think mehran has a 40L tank)
My alto does does 115-125Km with Ac in city easily. Without Ac i have once managed to get 160Km (i.e drove to the point when the car turned off due to lo pressure of cng)
Highway mileage 160-180Km, i once crossed 200+km on a single cng fill tank
However the mileage readily decreases when the scheduled service is 1500km away. i start getting 110km/tank. However after service (air filter change, oil change, cng tunning) mileage improves at once
as far as Maintenance is concerned, follow the manual. it runs perfectly. till now my maintenance included just oil change, air filter change, fuel filter change and got the brake pad changed at 20,000Km
I have an 2008 April released alto done 25000Km
overall nice experience