Hi Maya and Sanabas,
Just to add more detail to Sanaba's reply, it is fundamental to understand that speed/distance varies a lot with small and long distances. Let me explain with an example:
1) 240 kts TAS. This means the plane can fly 240 NM in one hour, and since 240 NM divided by 60 minutes equals 4, then the plane flies 4 NM per minute.
2) Now look at a short route, for instance 75 NM distance. You would expect the plane to cover the distance in 19 minutes because 75 NM / 4 NM per minute = 19 minutes of flying. The reality is more like 40 minutes. Both in game and in real life. Now use the 40 minutes trip time and multiply by 4 NM, which equals 160 NM --this is the optimum distance the plane can travel in 40 minutes. Finally, divide the travelled 75 NM by the optimum distance of 160 NM, it equals 0.47. The efficiency is 0.47.
3) Move on to a medium route, for instance 500 NM. You would expect the plane to cover the distance in 500 NM / 4 NM per minute, a 125 minutes flight. The reality is more like 155 minutes. As above, 155 minutes multiplied by 4 NM equals 620 NM. Divide 500 NM by 620 NM, it gives an efficiency of 0.81.
4) Try a longer route, 700 NM. Again, the expectation is 700 NM / 4 NM per minute, 175 minutes flight. Reality, 190 minutes. Do as above. Efficiency: 0.92.
Hope it is clear.