Originally Posted by ChrisAdams

Underdrive pulleys should really be last on the mod list.
Tuner is first, exhaust second, gearing if power is really wanted.
E-fans may help if you don't live somewhere really, really hot like I do.

The savings in power from under driving pulleys is obtained by slowing down your water pump, alternator, A/C or power steering.
I've seen kits for each one of these.

Slowing down the water pump is plain dangerous unless you live in the far North.

Slowing down the alternator may work out, if you don't have any electrical extras.
I have added more than a dozen power grabbing devices, if anything I may want to increase my alternator output at idle, the exact opposite of Underdrive.

Slowing down your A/C isn't a good idea south of Colorado.

Slowing down your power steering may be OK, unless like me you run bigger than stock tires.

So under driving is not really a great mod unless racing is all you do.
If it is a daily driver I would tend against it.
Chris