This average seems highly exaggerated, esp. for a new engine during its break in period, when the average remains at the lower side.
My Alto VXL AGS is 380 KM driven till now, and gives 18 Kmpl without AC.
This is how I calculate the real average.
Mileage before fuel tank top up = 20 KMs. Fuel tank top up = 27 liters. Resetted the fuel average, drove the car till it showed the last fuel bar, then the total mileage at that time was 380 KMs
Real average = (Total mileage - mileage before fuel tank top up) / fuel quantity used =
(380 - 20) / (27 - 7**) = 18 Kmpl
** Assuming 7 liters were left while the car showed the last fuel bar.