Individual part stability is based upon a median of all 4 statistics. So if you have Grip, Aero and Weight of 60%, and a reliability of 30%, your overall individual part stability is 50%. Imagine the Stability (in this case 50%) of the chance you will crash. 50 is not good. If all your parts have a stability of 50, you will have an overall Car Stability of 50. This is known in the industry as I Aint Gettin in that Wobbly Ass Car Fool.
The overall stability is a standard deviation and average of all the indivudual part stabilities from 100%. The closer to 100, the better. 50% is pretty poor. I wouldnt personally want to race with less than 90%.