Originally Posted by
DannoXYZ
A 5mm difference (170 & 175) is about 3% and makes a difference in rear-wheel torque if you're climbing a technical off-road hill. Going the other way helps you spin faster..
Though the same effect can be obtained by adjusting chainring or sprocket sizes. Ideally, you'd size the crank to your leg length, then use chainrings and sprockets to get the desired gearing.