You can't improve it further.
Vitz and Passo 1.0 have small engines but very heavy bodies.
So their fuel consumption in normal/congested city traffic is 10-12kml/L. Stop and go, acceleration reduces the avg.
On highways they are kings. Driving at 100-110 speed can give you 17 average on motorways.
11km/l is normal/good avg for city with our typical traffic.