Originally Posted by
Salubrious
Mark Stonich, who has shortened cranks for many years, maintains that there is no need to alter the gearing with shortened cranks.
Stonich recently provided my stoker wife with 151mm tandem cranks. She loves the new cranks. Crank formulas say 148mm for her. Her old cranks were 170mm. We can definitely tell the difference. She spins faster and has more endurance. However, her power on the steep stuff where our cadence drops below about 75 is definitely reduced and we're slower there. We might go faster there with lower gears, hard to say. Overall, we're faster over the road on long rides, largely due to her improved endurance. Reducing crank length reduces power at low cadences simply because one can only push down so hard without wearing ourselves out. Pushing down with the same force at the same cadence on shorter cranks will yield less power, duh.
Generalizing from our experience, use the formula to get crank length and use those cranks. We used Steve Hoggs formula of (leg length in inches)*5.48. Don't use overall height formulas or charts - silliness. Then check your gearing on the steepest pitches you are likely to encounter.
I doubt the OP will notice much difference between 170 and 165 - only 3% difference.