Hi John,
I don't know exactly how many teeth there are on the inner and outer gears (Sun and Annulus) but from the pictures I've seen I would estimate the gear reduction to be somewhere between 4:1 and 5:1.
I presume this setting is only required for speed calibration of the display unit, so you may have to alter it slightly if the speedometer/odometer reading does not match the GPS measured speed/mileage on your phone.
Alan