$ \frac{2.85\ mm^2}{1.75\ mm^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 \times r^2$.  Dividing the 2 values gives the ratio of the 2 speeds.  A gear ratio is them used to change the speed.