Ooops, I didn't notice the second page on this thread, but my last reply still holds.
tboner, to answer the question it looks like you will ask, I called the tire effect negligible because of what you said about the weights being very close and the distribution of weight in a tire. What I mean is that the vast majority of weight in a tire is not sidewall, it's all that rubber and belt that's on the road, and since I'm assuming negligible differences in diameter the overall wheel/tire combo, that weight is in about the same place, 16 or 18.
Ok, about this air pressure/sidewall thing. I can guarantee that the sidewall is only designed to deal with cornering forces, road generated forces, etc. Holding air is not on the list of worries that they are going to have. Have you ever seen high pressure hose? Heck, look at your garden hose, it's dealing with 30 or so psi and it's thin as hell! So 30-50 psi in something like tire rubber ain't no big deal.
Also,
get it out of you head that the sidewall holds up the car. The sidewall does NOTHING for holding up the car (other than containing air, which is what DOES hold it up).
tboner, what you said about the air pressure on the contact patch is exactly correct. If you measure it precisely and have the air pressure, you have the exact weight of the car. There was actually a question about this in the Sunday paper (Baltimore Sun) this past week in the ask the science guy section on the funny pages!
Whew, enough posting, I'm hungry... off to lunch
