Here's my personal experience. I have a 31" inseam and have been riding enough years to be able to feel some difference between crank lengths. 170mm and 172.5 are both comfortable and I can't really tell the difference between them. With 175's it depends on the situation. They're fine for me in any high-torque situation, such as mountain biking , loaded touring or hilly terrain. On flat land they're too long for me. I can feel my feet are going around in too big of a circle and my knees are going up and down too high and low. So I'd say crank length is a combination of leg length and intended use. From previous discussions on this topic (you may want to do a search, this subject comes up several time a year) many of the people responding say they can't tell any difference, so you may not either.