Would it be a good idea? a friend recommended it, but i don't know exactly everything they do, can someone explain?

 

Free HP
why not wont hurt

 

Waste of money. Save it for something else.

 

They basically just slow the speed down of the accessories so that they require less power to turn. Usually they are not bad but Ford designed everything to work on a hot summer day w/ all the accessories on and the AC blasting so underdrive pullies maybe detrimental if you encounter this a lot. Also, not all are created equal, some are better, some are junk.

 

+1