I don't think there would be such a big difference between the bike as a whole and the tire. Also, i'm not really sure you can simply hit the tire, conceptionally, it is not a separate object. As an analogy, you can't aim for the arm if you shoot a powerbolt at a person, because it comes from the inside.
Still, if we assume, that with some difficulty, you can hit the tire separately, i would say it requires some extra effort on your part (like a called shot, -4 dice), and would say that the threshold is not less than half of what it is for the whole.