The AI isn't consistent over all tracks. What I mean is 100% difficulty may be perfect at one track, and too slow another, and too fast another. I think the best way is to find the % that matches you for that car/track combo and go with that. Sometimes the AI can do stuff you can't like getting out of corners at ridiculously faster speeds than should be possible, but others they may be slower going into corners, etc, so I'd not worry too much about that.