but to answer your question, the pully would have to be smaller. pulleys work like gears. If you have two pullys of equal circumfrence for one revolution of the first you get one revolution of the second. If the second pully is twice the circumfrence of the first, for every revolution of the first pulley, you will get 1/2 the revolution of the second. That said, do your math and you can spin your alternator as fast or slow as you want.
