One of my bikes, which I bought used, has 175 mm crank arms. My other bike has 170 mm arms.

I looked up various charts, and the 175 mm arms are a bit on the long side for me (I'm 5'7" with a ~ 32" inseam).

Call me dim, but I really don't notice that much of a difference.

A few months back I was having a bit of an ache in one knee after 40+ miles, but I noticed that on both bikes. Come to find out the cleat angle on my shoe was a bit off, and adjusting the angle did the trick.

One of these days I'll probably change the crank on Dusty to 170 mm, the length I'm "supposed" to use, but I'm not in a hurry.