I am setting up a road bike for myself and have never adjusted the length of the crank arm. Am i missing something by not doing that?

If it feels good and nothing hurts, no.

A longer arm for example does not give your body more power, hence not more speed. It tends to reduce the cadence that riders find comfortable (my experience and many others here), which results in more speed for a given gear. With 172.5s I tend to ride a step higher which reduces my cadence and increases pressure in my knees. Not comfortable for me, I went back to 170s.

Doesn't it seem odd that cranks typically range from 170 to 175 mm on most bikes. A mere 3% variance, yet rider height and femur length vary by 30%.

21.6 percent of inseam for a tall rider equates to a 200-205mm crank whereas a short or short femur rider might need 145mm crank using the formula in the following websites. Only a few mfgs make even 180mm cranks and none of the big names make cranks shorter than 165. I am experimenting outside these ranges and it ain't cheap to do so. I think unavailability (long and short cranks) and lack of good data supporting proper crank length is why everyone just rides 170, 172.5 or 175 for the most part.

Bicycle Crank Length
Technical Q&A with Lennard Zinn: A question of crank length - VeloNews.com

The other problem with going to really long or short cranks is the frame design. Bottom bracket drop and chainstay clearance could be a problem for long cranks.....dragging a pedal in the corner or hitting the stays with the pedal or your heel.

30% variance in leg length doesn't necessarily imply we need that much variance in crank length. The real lever has many pivot points, the hip, the knee, the ankle, and the crank. No wonder it's so hard to figure out what an ideal crank length is.

I agree. I also do not see good data one way or the other. Who knows. The "studies" that I have read seem very simple. Max power for 60 seconds, etc. Aerodynamic studies proving shorter cranks are better on 40k courses.

My 200mm cranks shipped today, to be mated to Rotor chainrings. I have a hill that I train on all the time. Interesting if my performance changes on it but more importantly to me is the potential impact on fatigue at Rando distances and speeds for this thrasher/masher.

Yes, I'm more interested in fatigue than power. I doubt length affects power in any appreciable way.

Longer crank = more torque?

I don't think so. A longer crank can reduce the force requires by a little (though not necessarily the fraction of the length difference, since the lever has so many joints). But if you're pressing a little less hard, you're going to need to compensate somehow. The amount of power available is determined by the engine, not the transmission, and the transmission's job is to transfer power, not create it. If my thinking is flawed, please help me, because I'm just speculating.

In imagining what life would be like at the extremes, I'm sure I would not like cranks shorter than about 140mm. I realize this when I try a little kid's bike. Turning a high gear with cranks so short would be painful and might injure my knees. Cranks that are too long would bring my knees too high up at the top of the stroke, and that would probably be uncomfortable, but I haven't tried that. It would reduce force required, as mentioned, but it would also reduce the maximum comfortable cadence. What is the effect of that? I really don't know, but I suspect it could bring fatigue on sooner.

I think what I "need" is a 160 give or take on the drive side, and a 170 on the non-drive side. Supposedly, my right leg is 1.5 inches shorter than the left. But, I just use what ever came on the bike, usually a 170 or 175. After 59 years, my body has compensated for the difference in leg lengths; my spine resembles the letter "S"

I think a longer crank does give more leverage, all else being equal. But if you don't have the longer legs to push it, you'll strain and need to shift to a lower gear, cancelling it out.

It seems that crank length isn't a big deal for the vast majority of people since so many other things on the bike (leg extension, saddle fore-aft, etc) can be adjusted around it to get reasonably optimum power output and comfort. It is too bad that the larger companies don't at least take a token interest in the outlying sizes, though -- making smaller runs of cranks like 155mm or 190mm wouldn't directly help the bottom line, but loss leaders like that can help with goodwill and winning customers over to your brand.

170 mm for most bikes seems right.

165 for mini velos and folders to improve ground clearance and reduce pedal strike.

Yes, with the same foot force.

Ok, then you'll have some data. Share it?

ok. no problem.