Underdrive? Overdrive?

What's the difference between the two when they refer to underdrive pulleys and overdrive pulleys?

Well this depends on the stock size of your car's pulley in comparison to the size of the other pulleys... a smaller than stock would be an underdrive pulley an overdrive would be larger than stock.... basically you want to go with an underdrive pulley to get more horsepower from your engine.

what's the point of overdrive pulleys then?

not shure haven't heard anybody using an overdrive pulley, althoug using an overdrive pulley would increase powersteering response, more stable power output from the altenater at idle....basicly any accessory being over driven will do more work than normal which might not be good.

dude, overdrive is in auto trannies. it's just a higher gear ratio that allows for high speed cruising while saving gas due to low rpm's. i'm not too sure about the whole underdrive pulley deal, though. my best guess is that those pulley's are attatched to superchargers, and since smaller would take less power to spin, i guess they'd provide more power.

No... Your all a little off.

U/D pulleys are used to reduce accesory drag on the engine,such as the a/c and power steering pump.

Since the u/d pulleys are smaller than stock,they are a little easier on the engine because they are easier to turn than the stock pulleys,which makes more power and gives you a few mpg's.

As for the overdrive pulleys,it was all a dream...
Klick here to get an idea of what they are http://www.marchperf.com/ford_pulleys.html