The problem that you've stated, I'm sure that there must be a setting in the odometer settings, where you can select to reset the meter on every restart, on every refill, or manually reset. You can change that setting and I'm sure you don't have a problem with this meter resetting every time you start your car.
Secondly, about calculating the actual fuel average of your car, you don't need to do this practice of going again and again to the petrol station for refilling. Just top up your fuel tank to full from the best petrol station in your area, use it till the tank reaches half, again go to that specific petrol station to refill it again to full and simply note down from the odometer the distance in kilometers that you traveled divided by the liters it requires to refill the tank. The result will be your car's average kilometer per liter.
I hope it helps.
