I recently had to replace a bad tube on my bike and went to the LBS to get another backup. I have 700x28 tires and was looking for another one that had a threaded stem, but they didn't have any in that size. The shop owner told me I could use a 700x18/20 tube and it would be fine; he said it would actually be easier to install. I can't shake this feeling though that's too undersized for the size tire I have... is it? Even if it's okay, now I'm concerned that my CO2 cartridges would probably be too much for that size of tube (I believe it would put 120psi into a 700x28 tube).