What you want to look at is the sidewall shape of the tire when mounted on the rim you'll be using; the sidewalls of the inflated tire should have a pronounced curve, sort of like a light bulb. I would want any inflated tire to be about 1/4" wider than the rim that it is on, when meauring the tire across it's widest point. This curve in the sidewall gives the tire it's ability to absorb road shock and will minimize pinch flats.