The thread bump gave me the (sad) opportunity to read this missed thread in its entirety. Holy Moly what a cluster____ of speculation, misinformation, misperception and all manner of illogic. Carbonfiberboy is about the only poster making consistent sense. I'm sure he isn't the only one, but cripes. Listen. the percent change in ANYTHING between 170 and 175 is less than 3%! 2.94 if we want to get exact about it. Not perceptible. When you are talking about as gross an anatomical usage as an 80rpm cadence ... 3% gets quickly lost in the white noise of lactic acid overload. I don't doubt that for the first few tentative strokes with the new setup a person might think they have made a significant change, but once they start hammering it all just goes away. It really does.
A 10% change ... now you're talking. Why anyone would want 10% LESS torque to work with, I don't know, but if you are going to make changes, 10% is about the minimum that actually makes any real difference. For a 175mm crank that would be a 160mm crank. If you are doing it right you will also lower your gears 10%. Practically speaking you wouldn't try to lower each and every cog and chainring, just at the ends. Or you could drop down a gear (~13%) whenever you think about it. Chances are you'll do that automatically when you start to fatigue from having 10% less torque to work with.
As I have said before, the cranks we have are a compromise. Ideally they would be longer! All human powered endeavor is a need for speed. Our bicycles are descended from UCI sanctioned racing steeds that sought to allow constant high cadence pedaling for as long as possible. This meant keeping cadence constant through left and right hand turns at high speed. Much longer than 170mm and there is a real possibility of pedal strike in a corner and ... nobody wants that. Hybrids can run 175 because they run slower in the corners. But you are unlikely to see much below 160 or much more than 175 in regular production. Mainly because it makes little sense to have shorter cranks and 170/5 is the practical upper limit for longer ones without a frame redesign. Simplistic but true.
So why are there 155 ... 140. 100! Cranks? Because people want them. Misguided people, but they are a market. Fair game. Don't be that cyclist. Hold on to your money for something better than a crankset you don't need. The recumbent market is especially susceptible to the short crank thing. For 99% of recumbents standard cranks are just fine. The exceptions are those that enclose the riders feet as many only provide minimal clearance around the pedal circle. American feet just don't fit inside the toe boxes of European velomobiles with sporting pretensions. Sometimes cranks as short as 130mm need to be used but when you have the kind of drag coefficient the average velomobile does, what are a few (dozen) newton-meters of torque lost?
Look. I'm all for people doing what makes them happy. I just am dismayed that the reboot of this thread is just carrying the same old misinformation forward. At least know why you are doing something. Longer cranks are not the devil. Shorter cranks do not make you faster, or give you more endurance. At least not in the way most of you use them! Shorter cranks do not even really help your knees. Lower gears are a much better way of saving your knees than trying to limit the effective range of your knee flexion. TL;DR: unless you have a real good reason, and that is unlikely, the crankset sold with your bike, at least with respect to length, probably suffices. Piffling changes of 2.5mm and 5mm are just a colossal waste of time and money. Unless you are buying cheap crap cranksets and why would you do that?
I've yet to meet the production bike that wasn't grossly overgeared out of the box. If you are still using the 30/42/52 triple that came with your hybrid, or the 50/39 that came with your road racer you are not having a quality experience. The 30 (front) x 32 (rear) combination commonly found on most hybrids as the "granny" gear is about 27" which is also about the "gear'" that humans walk in. The low gear on my bike is 22 x 36. About 18". I don't expect a road racer to have an 18" low gear but if you guessed that the 39 (front) x 27(rear) low gear on many road bikes is too high, you'd be right. And you need LONGER cranks to wrestle that smb into submission, NOT shorter.