Join Date: Dec 2006
Location: Massachusetts, USA
Bikes: Mobiky, PBW, Jim Redcay, old Chicago Schwinns
The sprocket sizes are 33, 28/14, and 16. The nominally 12½ x 2¼ tires (ISO 62-203) are actually only 12" in diameter.
33/14 x 28/16 x 12" = 49.5" for direct drive.
The hub on mine is a Sturmey Archer SRB3 with 4/3 and 3/4 ratios for high and low, giving 66" for the high and 37" for low gear. Newer versions may have a Shimano Nexus hub which has slightly wider ratios for high and low.
I changed my rear hub sprocket from 16 to 18 teeth, which gets me up hills a little easier, and provides a little more clearance between the chain and the chainstay.