I would guess your MPG will increase in the sub 1mpg range. I doubt you'll see the cost come back in gains for a VERY long time, especially considering how light your stock pulley is (my 240sx's OEM pulley weighs about 9lbs compared to an Unorthodox 1.5lb pulley; now that's some weight loss).
The performance you'll mostly see in a lightweight pulley, which most people buy them for, is engine response, which I guess in a round-about way increases MPG because the engine doesn't work as hard to turn it's rotational parts. However, I never liked underdrive pulleys and prefer stock-size pulleys that are lightweight AND harmonically dampened (which 99.9% of aftermarket pulleys aren't).
Though I have heard some people say that using a lightweight (or underdriven, I can't remember) pulley on the A/C decreases power lost during A/C usage, so I guess that kind of increases MPG while running the A/C, but I don't have any first-hand experience.
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Originally Posted by Piwoslaw
Why are underdrive pulleys so much more expensive than OEM pulleys? Smaller production volume? Lightweight, hi-tech materials? An underdrive pulley is smaller, so it requires less material, and so would be cheaper and lighter.
Just my common sense.
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Simply put, because it's a "performance" mod and can be sold as such. I'm sure the automakers have good reasons for not using aluminum pulleys too (maybe fatigue?).
Quote:
Originally Posted by greasemonkee
Might buy a set of rod bearings to go with your underdrive crank pulley, especially if the factory had an isolator in the oem unit.
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Since he's probably not going to be spinning the engine to the upper RPM's often (I'm guessing), he probably won't have to worry too much about that.