Thanks Sheldon. Great page. One other point which may be helpful on it. There's another difference between "road" and "mountain" deraillers, and that's the angle at which the cage swings along the cogs. The thing that allows an XT derailler to be low enough to not bump the bottom of a 34t cog, when a 105 GS would bump into the cog. It may not be worth mentioning this difference, because the only thing it affects is max cassette tooth capacity, but it is a technical diff. between "road" and "mountain" deraillers in the Shimano family. (Of course, a "mountain" derailler will shift fine with a straight-block "road" cassette, even though the derailler pully ends up further away from the cogs as you move to lower gears than would be the case with a "road" derailler.)
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"c" is not a unit that measures tire width