Tuning errors.

well people i’ve been trying to tune up my car and while i was watching the brake distance, top speed and others, i realised that if i moved something like, lets say, tires preassure from 21 to 20 and then to 21 again, the number in the break distance from 97 to 0 km/h wasnt (lets supouse) 120M to 123 and then 120M again, no no no
when i came back to the same config i got a total random number like 250M. i got out of tuning without saving changes and i allways get a different number, and then i went to gear box config to put up those raptor gear and i got an ERROR msg! ERROR!