One of my concerns when considering a 30 spd vs a 20 spd bike was the extra deralleur length required for a 30 speed chain and would this slow down shifting or make it less determinate. I was also interested in how useable all the gears were. I came up with the following table I'll share. My rules for useable gears were as follows. If I'm on the inside chainring then the outer two most gears on the freewheel would not be useable. Likewise on the outermost chainring the inner two most freewheel gears would not likely be used. On a thirty speed I adde a rule that on the middle chainring, the first freewheel gear would not be used and the outer most freewheel gear would not be used.
The unuseable gears are highlighted in yellow. The gears that are redundant are highlighted in blue.
What is interesting is that right in the mid range the useable gears jump by as large a factor as for the 20 speed bike with the finer gradations occuring near the top or the bottom of the range. Opposite of what I would want.