low mileage could be due to a lot of things..
1- get your tappets adjusted by an expert mechanic. (very important for cuore specially on CNG). Do this every 6 months or 10 K kilometers
2- get distributor serviced
3- ask mechanic to check gas flow screw on cng kit. may be its opened too much. After that adjust the idle screw.
4- use/change a good engine oil (I use Havoline Formula) and always change by or before 5k Kilometers. Also change oil filter at every oil change.
5- clean air filter once a week. perferably use genuine daihatsu filter
6- Change the ignition "point" after every 10K Kilometers. Use a genuine one from toyota dealership
7- Check Spark plugs. You can get very good mileage from stock plugs, just get them replaced/cleaned timely.
After you have done all this I am sure you will get a much better average.
btw my Cuore gives about 120 KM with A/C in city in about 7.5 Kg of Gas
and about 160 KM with A/C on Lhr-Isb Motorway
I still think this is a bit low and I am thinking of getting the tappets adjusted in a few days.
( please always mention the amount of gas when you calculate mileage. There is no such thing as a full gas cylinder)