I have an app that I use to track expenses - gas, service, insurance, etc...most importantly kms/liter.

I just checked the online tech specs of the car, which list the fuel tank at 40 liters. The problem is that in the app I defined the fuel tank as 50 liters because when I fill up the tank I usually put in around 45 liters and never wait until the tank is empty to fill up.

What's going on? Is it 40 or 50?

(The app shows that I get 11-12.5 kms/liter, which I think is normal.)

 

50 liters is 13.2 US gal, which is correct. There's actually a little reserve past this.
40 liters is only 10.6.

 

Since mpg or kpl (kilometers per litre?) Are based on distance divided per fuel added....I would not worry....
I would suggest and caution you not to run too low on fuel...reasons beyond keeping that pump in the tank partially submerged to keep it cool...things like security, if some type of disaster or emergency happens and you are suddenly trying to find a open fuel stop.....
Many folks learned that here in the US when a regional blackout occurred a couple years back...and folks could not buy fuel for half a day ....sucked if you were so low you HAD to buy fuel to drive home cause you were REALLY LOW....folks were litterly stuck..if I recall, your neck of the woods is...er...interesting at times.
My suggestion...
As long as you fill up the tank till it clicks off or you combine partial fillups with the distance traveled till the next complete fillups Any software should not care if it is any good and written right...so fill up at 25% or more!!

PS
The tank is plastic... So it does flex like anyfuel tank, and a set volume is not included in the volume calculations to allow for expansion of fuel as it get warm on a hot day...yes..it can expand by a couple % so about 10% of the tank is left empty to allow for this ...

 

Never a dull moment.

I usually fill up when the red line lights up (1/8 of a tank left) - just to be safe. Thanks.