not expert here but have experience of using idsi from past few years.
please do following check on cars. good engine oil like of that zic , castrol magnetic but of 10w40 , not thicker then that ! check on air filter / oil filter / fuel filter / plugs (replace with stock ones) / 30psi air pressure in tires, use light foot + Ac on (close windows), its all about driving style, i used to get approx 20- 21 km/Liter on long from my manual idsi 06 and around 16 km/Liter on other vario ! on long. driving between 100 - 110 km/hr is ok to get max mileage and cover the long distance, going on 90km/hr is somewhat more patience demanding
Good luck, no doubt, city is best in affordability in its league .....