All motorized vehicles in FS19 get automatically refueled each time you start the game if they have less than 10% fuel in the tank.
This means, you exit and reload your savegame with less than 10% fuel in the tank you get filled up back to those 10%.
So if you don't play a long time in one sitting but save and reload more often its basically impossible to run out of fuel.
This isn't just unrealistic, this automatic refueling also costs twice as much as regular refueling. I've been annoyed by this feature from the time I first got told it exists.

So here is the "fix" for this "problem". No automatic refueling. Have fun!