I don't know if someone already covered this so here goes. There is a reason why aftermarket companies offer pulley "sets".
They will all include a crank pulley. Usually this is smaller than stock so EVERYTHING else hooked up to the belt is slowed down the same amount. But the alternator (or others such as water pump) may be able to run even slower or it may require a higher speed than the crank pulley's ratio to produce enough output. Therefore they make a pulley to make up the difference.
In our case the crank pulley is probably matched to the water pump, power steering and A/C. The optional alternator pulley is bigger than stock to slower it down even more.
AKM
'98 Dakota R/T (JBA, Transgo, ported T/B)
'89 Mustang GT (347, Vortech S-trim (intercooled), Griggs GR-40, etc.)
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