Depends on your measurements and preferences, so hard to say. In general a shorter crank will enhance spinning, a longer crank will enhance torque. Who knows maybe a 185 or 165 or maybe a 172.5 or 177.5 might be better for you?
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Size and inseam would help. I'm 5'7" with a 32" cycling inseam (30" pant leg). I ride a 172.5 xtr 970 crank and it's perfect. I've also ridden 175mm without trouble but the shorter crank feels better.
Unless you are on the extreme of one side or the other of height/leg length, I would not sweat it and just get a 175mm crank. It is probably what you have now. There is probably a fit formula out there that can help you determine the ideal crank length. 2.5mm is less than the thickness of two dimes stacked on each other. 5mm is less than four dimes stacked up. It is only a 2.8% change in length going from a 175 to a 180mm crank and a 1.4% change going from a 175 to a 177.5. Also if you tweak a crank arm, I would guess odds are in your favor of finding a 175 replacement. The flip side is if you want to try a different length out, go for it. It is probably not gong to be a drastic change from what you have now.
Unless you are on the extreme of one side or the other of height/leg length, I would not sweat it and just get a 175mm crank. It is probably what you have now. There is probably a fit formula out there that can help you determine the ideal crank length. 2.5mm is less than the thickness of two dimes stacked on each other. 5mm is less than four dimes stacked up. It is only a 2.8% change in length going from a 175 to a 180mm crank and a 1.4% change going from a 175 to a 177.5. Also if you tweak a crank arm, I would guess odds are in your favor of finding a 175 replacement. The flip side is if you want to try a different length out, go for it. It is probably not gong to be a drastic change from what you have now.
The range of crank sizes always makes me laugh - 170 / 175 / 180 - that's a whole 10mm or 1 cm. Enough to move mountains? Probably not.
Yes, but the circle that your feet follow is twice that diameter. 10mm difference in crank lenght is almost an extra inch diameter and almost 3 inches perimeter distance. When I road USCF I could tell the difference between 170 and 172.5.