Usually in city vitz/passo should give 10-11km/L with AC. Stop n go traffic.
But if you have really pathetic traffic and use very hard pedal it can drop a bit further..
To get good avg, first of all use 0w20 engine oil.
New Air and AC filter.
Clean throttle body, intake manifold.
Check condition of spark plugs. Change only if deteriorated.
Fill some hi octane. Go on highway, and press full paddle go really hard on it and speed it up for like 50km. It will Clean up catalytic converter.
Use liqui moly fuel system cleaner.
Should improve things.