[McTaff]

Hey champ, that's a good summary, but I'll break it up a little to explain a little more. This might help others to decide what they want.

Drive pulley on a belt-pulley system:
We will call the source pulley of the power the "Drive" pulley. This is the pulley that will supply the force to turn the belt.
We will call the receiving pulley of the power the "Slave" pulley. This pulley is turned by the belt and thus powers secondary devices.

Each slave pulley in the system can be treated seperately as a lever and fulcrum system. Force applied is at the Drive end, and work done is at the Slave end.

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On a per revolution basis, increasing the size of a drive pulley will increase the number of times the belt turns for each revolution of the drive pulley.

This decreases mechanical advantage (like moving your fulcrum closer to the applied force), but increases the distance travelled (the other end of your lever moves further). This means it will react less favourably to high load situations, as the transmitted force has less mechanical advantage, like trying to use a shorter lever. (Not really a problem with drive belts in cars, though)

Decreasing the size of the drive pulley will decrease the number of times the belt travels.

This increases mechanical advantage, but decreases the distance travelled.

Now remember that doing this to the Drive pulley changes the "fulcrum" for ALL YOUR SLAVE PULLEYS.

To re-adjust each slave pulley INDIVIDUALLY, the opposite is now in effect, because you are changing the other side of the lever. (By making them smaller, it is like a "Claytons making the drive pulley bigger".) This means you need to reverse the formula.... i.e:

Making the Slave smaller has the same effect as making the Drive bigger, and vice-versa.
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You need to work out the ratio of Drive change and apply it to the Slave and bring it back towards 1:1 maybe half way. If you went from circumference X to circumference Y on the Drive, work out the ratio X:Y and reverse-apply that to the Slave pulley.

That will give you the size of the Slave pulley you would need to return it to the original specs.

All you need to do then is to pick a size maybe halfway between the stock one you have, and the size you'd need. (Unfortunately this is a black art, and it's rather like inventing a recipe... you have a fair idea what the ingredients do, but no idea what the end result would be like.) This would be a rule of thumb. But I'd say halfway between them would balance it out - if it's close but not quite perfect you'd already have a fair idea because you've already crunched the numbers.

Hope that helps the people who like to fiddle.