I wouldn't bother. My bikes have crank arm lengths of 165 mm (tandem), 170 mm (touring and folder), and 175 mm (road) and I can't say that I really notice much difference between them. Yes, I do tend to pedal at a little higher cadence on the tandem - but I'm not sure that it makes me any less or more efficient than the lower cadence with longer cranks on my road bike.