Jaun this is almost another how long is a peice of string question.
first which aircraft are you talking about, the perfoemance of an F-16 is somewhat different from that of a AN-225.  So to really answer your question we need to know which aircraft you are talking about.
Also, there are differences in the way the same aircraft from different designers perform, for example you can download 20 diferent F-16's and none of them will perform the same.
The only way to fix it is to learn to design your own aircraft or at least learn how to tweak an air file.