Well, here it is, Stupid Question #1. What exactly is a "Underdrive Pulley System"?
 

Just started doing some Online Window Shopping for some minor (both in cost and difficulty) performance mods for my '05 GT, and a co-worker suggested I take a look at Roushs' website. This was something that caught my eye.

They eliminate some of the drag caused by the pulley's to free up a few HP.

Also you can find them alot cheaper than from the Roush website.

give yeh a couple mpg too

Don't bother. UDP's aren't worth but 3-4 PEAK hp and give you almost nothing under the curve until you get to the top end. They do screw up charging in many cases (got a strong stereo, cancel your plans for UDP's) and make life more difficult if you decide to go the blower route later.

Average dyno verified HP increase I saw on more than 20 cars was less than 4hp peak and 0 average. You will see a small MPG increase but not enough to say grace over.

Good luck. If you want performance mods for your 05, get it dyno tuned and slap on a K&N FIPK, remove the filter from the snorkel. You should see 20-30rwhp gain and that's a real gain you can see on the dyno and in the SOTP meter. NOTE: 05's are different in that nearly any mod that will make a good power gain requires some computer adjustments. After you pop for the intake, do the gears. After that... you should be pretty happy for a while.