It is not a matter of relative speed, but of the relative angle that you try to get a radar contact from the KC. You see, if hit from the side (abeam) it will bring you a much stronger echo than if you illuminate the tanker right from astern on head on. If the simulation works correct and these physical rules were considered by the programmers, you probably will see a KC 130 from 80-90 km when you look at it from the side.
I'm sorry but this is wrong. Doppler-pulse radars depend largely on relative speed, and the radar cross section of a plane depends on more factors than just the size of a surface returning the radar signal.
Greetz,
Raploc