My cousin having 2010 model vitz CVT 70 K driven and its giving 16~17 km/l within city with A/C. Its average on highway is almost 18 km/l at the speed of 120 km/h. All with regular petrol and without any hi octane.
Change the spark plugs, use ZIC A+ engine oil (5w-20), and instead of using full hioctane, just add is 1/3 or 1/4 with regular. Keep its airfilter clean and fill the radiator with nice coolant.
It will help to increase your mileage further.