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While I agree with your statement that the rider will get used to whatever crank length they have and there is no mechanical efficiency as long as gain ratio is constant I still believe there would be an optimum length for the rider based on how their body has been formed (muscles, height, leg length etc,.) and the different RPMs involved. Even if that optimum is just for comfort.

Again, probably so little difference that it is not an optimum worth trying to achieve with so many others factors to worry about.

I'm with you on this. I run 165s on nearly everything, as I'm short. My SS MTB has 175s, which I love for the use. But I could'nt see myself being 100% comfortable with 175 on the road. I'm probably an extreme example though.