In article 1134263285.640573.286370
@o13g2000cwo.googlegroups.com,
says...
Another dumb question......Is the diameter of a pulley measured by the
overall outside diameter or the smaller inside diameter where the
bottom of the belt rides?


Not dumb at all, unfortunately the answer is neither the
bottom of the groove nor the OD. The pitch diameter, which
is what you need to know if you're calculating ratios,
should be close to where the cords (which are molded into
the belt at its neutral axis) lie in a v-belt properly
fitted to the pulley. If the pulley can be used with more
than one belt section, both A and B, for example, it will
have a different PD for each.

Ned Simmons