underdrive pulley ratio

[hamondale]

When you go to an underdrive pulley(s) on the crank to slow down your water pump/alt/AC, typically about what percentage is the reduction in diameter of the pulley from stock? I can't find a simple single sheave v-belt one for my 3.4, thinking of having one made up. Thx.

[Doubt_Incarnate]

it varys i think 25% reduction is common