Originally Posted by
Moisture
A 2.5MM difference in crank arm length is damn near going to make no difference. You'd notice more of a difference upgrading to a higher quality, lighter crankset or even changing out your gearing.
Crank arms need to be proportionate to your inseam and how much power you output.
The cranks I went to are 170, not 172.5. I wrote that by mistake. Even with a 2.5mm difference, you are changing more than the length of the crank, you are changing the circumference of the circle the pedals travel. You also have a 5mm difference total top to bottom.
Leg length alone isn't all that matters. A shorter crank also helps decrease the total bend of the knee at the top of the stroke, which can help people with limited flexibility due to injury, or other issues. It also helps put less stress on the knees.
I will edit my original response to correct the length.