The actual mileage claimed by Hyundai for Grand i10, which is also specified by ARAI, is 18.9kmpl. However, it differs on highways and city roads. Furthermore, mileage depends a lot on the driving style and the condition of roads where vehicle is being driven.
That's great if you are planning for a new car. It might interest you to know that Hyundai is planning to launch the facelift version of the Grand i10 sometime this year around August/September. The Grand i10 facelift is also likely to get AMT (Automated Manual Transmission). So if you are planning in 1-2 months, we suggest you to wait for its new facelift version and other new upcoming cars.