This app does not show mileage from the car's computer, rather you add information in it, like, how much fuel you added, how many kilometers you drove etc. Its more of a log of fuel being filled. It calculates fuel economy based on your inputs, whenever you fill the complete tank.
After driving 386 km, you should refill the fuel tank. Simply divide the kms by the number of litres filled and you will have your fuel average. This is the most reliable method. The fuel average shown on the meter by most cars is often misleading. In a 2015 model like the one I owned, generally when the fuel gauge reached the redline, it had used around 27-28 litre, and when it reached 'E', it would still have around 6-7 litres in the tank, that is, it had used around 30-31 litres. If we consider it to be 30 litres, than your average will be 12.8 km/l.
Tuning doesn't affect average unless there is a problem with your car. Your driving style matters more. Driving at 150 km/h will reduce the average drastically. Similarly, drive below 3k rpm, i.e. 108 km/h, and see the wonderful fuel average you get. Also note, most Japanese manufacturers' vehicles show 10% higher speed on the meter, so when their meter shows 110 km/h, they are actually around 100 km/h. V2 shows the exact speed, i.e. 100 km/h. So when comparing average at a particular speed with another car, keep this in mind.
Hope I answered all your questions.