Let me try to explain this, for those who are still following.
A rim, and tire is technically an annular cylinder. You could calculate the moment of inertia by calculating the annular cylinder about the symmetry axis. 1/2M(R 1over2+R 2over2) I believe is the formula, kinda hard to put it in a post like this, but hope you get the idea. You have to keep in mind, you still wouldn't be correct even then, do to the altering forces of the vehicle it's self suspesion, weight, eta. then you have different road surfaces, geometry of the road surface, that act upon the annular cylinder (AKA wheel and tire) or in other words there are real world conditions that have effects in this equation.
Red_Zed figuring out the moment of inertia of a 18" and 19" rim is great if you're in a class room, and you're solving this in a controlled environment. So in theory yes, you are correct, in caluation I would need to do the math to make sure myself to say other wise. The reason you aren't correct though is clearly you're not including the full circumference, width of the tire on either given rim, their total mass, or any of the other factors for that matter in this that play huge factors. People don't drive in class rooms, or completelly controlled environments with prefect surfaces either, not to mention there is something attached to that wheel, call a vehicle.
Red_Zed look more at the big picture, and not just one part, that's why this Bull$sh!t really has no relevance to "why 18'' wheels more popular on track?" like I said before. This is a case of people being stupid, and aguring over something that is completely superfluous to the topic.