I run 20-16 with 165 cranks and it feels pretty much perfect in my opinion. I have also use 175 cranks with this ratio and it felt good too, definitely felt a little better for trialsy stuff, but to be honest you can’t feel much difference once you get used to it. I may be going back to 175 soon so have been thinking of going to 20-15 just to give a gear with a bit more speed, but while still getting plenty of torque but I probably won’t bother. On a side note how do you work out your gear ratio, when taking into account the wheel size and crank length?