Originally Posted by noglider

Years ago, I learned the tire industry's method for rating sizes. They flattened the tire out, measured the distance bead-to-bead, and then divided by 2.5. This will surely yield numbers other than the inflated width, depending on aspect ratio, among other things. I don't know if they use the same rule these days, but if they did, I would not be surprised. That's why it's good to get anecdotes about how wide a tire actually is, if your clearance is critical.