My inseam is 30.75" and cranks are 170mm. I have found that during winter backpacking wearing heavy insulated boots and snowshoes. the muscles that raise the legs can tire. I used to do leg exercises for this with 10 pound leg weights. Those same muscles raise the legs during cycling. On long rides, when I begin to tire, I've noticed that the leg at the bottom of the pedal stroke begins to relax and becomes dependent on the down stroke to raise it. Consequently, I guess that a shorter crank would save the legs just a bit so they would not need to flare out at the top of the pedal stroke.