Originally Posted by
ThermionicScott
What?? That is completely wrong.
You're right, I made a mistake there. Though it's not completely wrong that you can make this stuff work together, I exaggerated compatibility quite a bit.
You can use 8sp with 7sp, and 7sp with 5 or 6.
Spacing per gear:
5 & 6 sp - 5.5mm
7sp - 5mm
8sp - 4.8mm
You can use 7sp shifters with the alternate cable routing described by Sheldon Brown
here to pull more cable. (Shimano also publicized this at the time)
You can use the same trick to use 8sp with 7sp - though they're close enough you might be better off with standard routing, depending.