Most 3mm filament extruders have some kind of gear reduction. Many 1.75mm extruders are direct-drive / ungeared but some do use gears. What kinds of reduction ratios are suitable or optimal?
(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.