I decided to give 170 cranks a try for the past three months, I have always been on 172.5. Initially I like the feeling that I could "get over" the gear a bit easier and coming out of the off season it was kind of nice. However, I found my bike splits in our short course season (i am in Australia) were lacking behind the guys I usually ride with and I felt my fitness was fine. I've decided to go back to 172.5 and right away felt I could get more power and ride at my old cadence and bike split times. There was no real reason I went shorter initially, I was just following a lot of discussion on shorter cranks. In issue for me is I started riding 22 years ago when I was 13 and we had 172.5 cranks and I've always had that crank length. I ride about 85-90 cadence and am not really able to change that as its just what i do, so I am not capable of change, I'm too stuck in ways so 172.5 is where it is at for me.

Also a few years a go I bought a new road bike, it came with 175 cranks, I lasted about 3 months on those, I actually almost wanted to give up cycling as could not get comfy on that new expensive bike, then I put some 172.5s on and I was happy.

I am actually amazed in both instances (longer and shorter) that 2.5mm was even noticeable and annoyed me so much.

Anyways, it just shows this crank length stuff is really personal.