If the fan clutch is working properly, it is my understanding that the fan should be more or less free wheeling if the engine is cool enough, so it shouldn't waste any energy. Another consideration in calculating fuel usage is the efficiency of the electrical fan. The electricity isn't free, it has to made by the alternator, which isn't close to 100% efficiency. Add to that the fan motor itself isn't 100% efficient, and you are probably pretty close to the same energy usage as the engine driven fan. Would be interesting to do the calculations to figure out which was more efficient, but I think I would put my money with the engine-driven fan. Less "middle-men" so to speak in the energy conversion.

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Keep everything as simple as possible-but no simpler--Albert Einstein