## 1 Answer

(2.85mm)^2/(1.75mm)^2=2.65 which is the reduction you would want assuming a 1.75 is direct drive.

In this answer I am comparing the amount of filament flowing though the extruder by using pi r^2. Dividing the 2 values gives the ratio of the 2 speeds. A gear ratio is them used to change the speed.