Five millimeters of crank length increases the distance the pedals travel in every revolution by a little more than 31mm.

Going from 175mm cranks to 165mm cranks decreases the distance the pedals travel by almost 2.5" or 63mm.

Bottom line, 165mm cranks can spin 6% faster than 175mm cranks.

(63mm / 1037mm = .06, or 6%)

What kind of additional effort do 165mm cranks require in comparison to 175mm cranks?

Six percent.

(Weight times Arm equals moment; or, 10kg X 165mm = 1650kg; 10kg X 175mm = 1750kg; 1750 - 1650 = 100; 100 / 1650 = .06, or six percent)

So, by reducing your crank length by 10mm, you can increase your spin by six percent, but you will also increase the effort by six percent.