car mileage is excellent

I am bought a new car and ihave the datsun go+ aniversary editon the exterior of car is excellent company have to think about the interior of the car because interior of car is not high classs but ok and about mileage one person drive my car the car gives only 11 kmpl but after i drive the car for 600 km and car gives 18.5 kmpl mileage on highway so in my thinking car mileage is only depending on how the speed of car is running so please maintain the speed of car 60- 70 km/ hour then car gives good mileage and don't run the car at high speed and i am good with my decision to buy datsun go+ aniversary edition