I can't say what'd be best for you, but I'm your height and inseam and I can't imagine being happy on 175s. I ride 170 arms on my road bike and 160s on my tri bike. I had 165s on my tri bike for the past several years, but switched to 160s a few months ago when I bought a new crank. I can get a slightly better position now. Just my .02; ymmv.

Depends on your bike fit if you can gain from switching to to shorter cranks. If you're in a more aggressive position you can open your hip angle at the top of the stroke by using shorter cranks and possibly be more comfortable and produce a little more power. At your height you can probably go with 170 and even lower 165s. You will need to adjust your fit coordinates a little bit (seat higher, and possibly adjust reach too).

Take a look here:
https://www.youtube.com/watch?v=3BrhdFeJMOI

Thanks guys! I am going to try out the 170 then to start. The 172.5 on road feels fine but I have not tried a 170 and the idea of more comfort will always equal more power so I am good with that one.

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“Two things are infinite: the universe and human stupidity; and I'm not sure about the universe.” - Albert Einstein

I used to have a mechanic for a team I raced for that was the Cuban National Track champion, or so he told us (I do know Castro put him in prison for a while, random) but anyways, on my road bikes he always had me on 175 (long legs, high cadence) but he always had us on shorter cranks for TT and in my case, triathlon. He always said it was for two reasons, the bike fitters liked that we could get more aggressive and open up our hip angles (as mentioned) but also it's a take away from track racing in that it lowers the torque on knees. I'm not a doctor so I don't know if that's medically accurate but I did like the 172.5 on my TT/Tri bikes because I didn't feel that my knees were taking a beating while putting out higher watts over shorter distance in a more aggressive position. Just food for thought. Go shorter on the TT/Tri cranks.

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Todd

I have trouble seeing any value in going longer for TT efforts if you plan to ride aggressive. However, it also seems that it might not make that much difference.

What i do like is that with shorter cranks, you can effective lower the stack on the bike as you'll end up raising the saddle. I also feel like with less leverage, I tend to ride more evenly. But that's probably mostly mental. Ultimately the velocity of your foot remains the same, so there's very little change overall.

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TrainingBible Coaching
http://www.trainingbible.com

Not sure if this helps but im 5'6 28 inseam
I have two Tri bikes
My training bike is a 170
My racing bike is a 165

I prefer 165 I tend to be able to spin faster and it feels more round or circular than the 170

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Rain*

My wife and I are 5'8" and 5'9" and we are both happy on 165mm cranks. 172 on the roadie.

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A false humanity is used to impose its opposite, by people whose cruelty is equalled only by their arrogance

Just adding some more info to help:

5'4", 28" inseam

My sweet spot is 170mm. I've been running 165s recently and have run 172.5 before just to try it out. Cadence is typically around 80ish and I felt like I was spinning a bit more than I physically prefer on the 165s.

Also 5'6" with ~29" inseam. Originally had 165's on tri bike. Tried some Cobb 145's. For me they were a large increase in comfort along with a more comfortable high cadence. I think it's due to the more open hip angle and corresponding reduced interference between leg and abdomen. Felt strange at first, now completely natural.

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Remember Luddites are people too...

I tried sizing down from 172.5 on my TT bike and didn't like the feel. I found it surprising that 2.5 mm was actually a noticable difference, but it was. I rode a number of back-to-back TTs and the shorter cranks weren't faster and I didn't like the feel, so I ride 172.5s on all my bikes.

Given that nobody has shown a power loss for shorter cranks, I would try them and see if they feel better or address a fit issue. My guess is that +- 5mm a change should be more about feel than some theoretical advantage.

While I have never seen anyone else recommend this, I find that cadence is a good indicator of fit. If you find that you need to decrease your cadence (or shorter cranks help you maintain a faster cadence) than go with the fit that makes spinning the easiest.

the only safe answer is that no one knows, I think. There was a study done long ago which showed, in summary, that crank arm lengths made no difference in power production, and the cycling industry basically standardized crank lengths. It would be cost inefficient to manufacture 10 different crank arm lengths and fit them to athletes. I found a 180mm and I installed them because I wanted to, mostly because I like to ride at a cadence of 80 in heavier gears, that is where I get the most power. Florida is generally flat as well. People frequently criticize the longer crank arm I have, when we are really splitting hairs in terms of MILLIMETERS. So, who cares?? In an ideal world, where costs and production didnt matter, your crank arm length would be determined mostly by your inseam length. You have long legs for your height, so you can go on the longer side, and you obviously have no issues with cycling cadence, you cycle like Lance 90 and above. I actually believe the slight difference in crank length makes no difference at all. People are quick to discredit the opposing opinion. Longer cranks aren't bad...