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Yes, but probably by an amount so small that the price of the pulley and the amount of work to install it would never pay itself off.

 

The best reason to install it would be to help your acceleration by reducing the rotating inertia of the engine/accessories through each gear run-up (even then the gain is small).

 

Any measurable fuel mileage gains you'd see would be during city driving because of the abundance of rising RPM'S during shifting. Steady-state highway cruising would show almost no mileage gains.

 

If you live in a cold climate, the underdrive pulley could allow a battery discharge when you have a lot of accessories on at a stoplight or during slow driving.

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i posted about this a few weeks ago and talked with someone who had one installed and ended up selling it to get a regular size lightweight aluminum crank pulley because it was causing charging issues. (i think that's why they are cheaper because the demand for them isn't there).

 

i'm going with a full size lightweight crank pulley to avoid any issues. it will still be light weight so you'll get almost the same gains as you would with an underdrive pulley. fullsize pulleys still weigh like a few pounds less than stock and barely anything more than an underdrive.

 

another reason i like aluminum pulleys - ONE PIECE. these subaru pulleys with the harmonic damper on them piss me off as it's a failure point, the very fine rubber ring can detach from the metal. the stock pulley is really two seperate pieces of metal seperated by a very thin ring of rubber. that rubber goes and either your pulley falls apart or you'll have a mysterious charging problem where the inner part of the pulley connected to the crank turns faster than the outside part of the pulley...it slips. very annoying and a failure point that i have no need for.

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