Shimano presently offers their road cranks in 4 sizes ranging from 165 to 175mm. Think about the fact that this small 1 cm range is designed to serve humans that are 5 ft tall and those that are 6.5 ft tall. Obviously there is a lot of leeway here. Many years ago when I was buying stock bikes, a size 56 frame sometimes came with 170 cranks and other times with a 175, there was no 172.5 at the time. I became accustomed to 175's on my mountain bikes and so over the last 20 years when I build a frame up, I use 175 but I'm sure that I can't tell the difference in 2.5mm,.... 5 mm maybe?...wouldn't want to bet my life on it. A few years ago I changed out a bottom bracket for a friend who had over 20,000 miles on a bike after just a few years, and when I went to remove the cranks, I noticed he had two cranks arms of different lengths and yet he had never realized the difference. Ha