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Tire rolling resistance is way, way more dependent on the make and model of tire than on size. In other words, tire size plays a small role in RR.
In other words, there is no point is going smaller in trying to achieve better fuel economy.
PLUS - the diameter of a tire will affect the overall gearing - larger being better. So it is good if you can fit a larger diameter under the fenders (and that is larger overall diameter, not larger wheel diameter!)
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