I found the difference in cranks does affect how I spin. On my main road bike I felt like at speed I wasn't smooth, so I switched from 175 to 170 cranks and now I can spin better. But I did discover that those big hills are harder to climb. I have a cyclocross bike which I ride on the road and on trails with 175 cranks and it feels fine to me. My mountain definitely is better for me with longer cranks, have more power at slow speeds.