Underdrive pulleys slow down alternator rotation and can result in reduced battery charging and headlight dimming, particularly if you install an upgraded audio system.

....like I said, I have not experienced any worries with 100W RMS. I guess it depends how much upgrading you are expecting to do in the stereo department. Again, I stand by this mod as a great upgrade.

UR notes that the retreived power comes not just from underdriving the accessories (which requires less torque, hence the gain), but also from reduced inertia (i.e. less rotational mass) due to lighter material. In fact, the hp you gain back is more a function of the reduced weight than it is the smaller diameter.

this car also being a daily driver; how's the effectiveness of the a/c, heating, defrost? negligable difference? will upgrading the battery help any?
-meGrimlock

Over 15,000 miles on my car since dropping in the pulley, and I've never had a problem. Stock battery is fine, because the impact on charging and other accessories is not as drastic as you would think.