brother I have Alto 2007 at Lhr. with the factory fitted CNG Kit. I have same 135 km average with Alto(with a.c). The inter city average is 150km with the same Cylinder.
The thing is Alto and all other car manufacturer in pakistan use old engine technology. Now the engine are much more fuel efficient.
The advise may be to go to "Enercon Tunning Center" , (Govt. of Pakistan) they can adjust the best possible average.