I was just designing a medium airplane with 2 engines and noticed that it has an operational range of 25, no matter if I give it an 18k, 25k, 40k, or 60k fuel tank. Then I noticed that the amount of kilometers actually increases with the fuel tank.
These two airplanes are the same design, except the left one has a 18k fuel tank and the right one a 60k fuel tank.
See that the 4991 km on the left divided by 200 get you almost exactly 25. Dividing the 6292 km on the right by 200 gives you more like 31 hexes.

When I designed the same plane as on the right of the previous picture, but give it a weaker engine, it will now suddenly say the correct amount of hexes, but only when the plane is not able to take off:

