If I recall correctly, only the “race” or “rally” class of certain parts (such as brakes and suspension) allow tuning. It would be nice if lower classes (“street” and “sport”) of parts also allowed tuning.
For example, I would like to be able to set the ride height of “sport” suspension and/or the front-rear balance of “sport” brakes. If this would not be possible in real life, then I would understand leaving it out of the game.