speedo gear
There is no drive gear per say. The output shaft has the teeth cut on it to drive the speedo gear. Here's a speedo gear chart. Sorry about all the dashes and periods. That's the only way I could get it to post anywhere close to right. If you have trouble making it out ask me and I'll help.
Rear Tire Diameter
...............24"--25"--26"--27"--28"--29"--30"--31"--32"--33"
Gear ratio
2.76 teeth 30---29---28---27---26---25---24---* * *
2.94 teeth 32---31---30---29---28---27---26---25---24---*
3.23 teeth 35---34---33---32---31---30---29---28---27---26
3.55 teeth 39--38/37-36---34---33---32---31---30---29---28
3.73 teeth 41---40---38---36---35---34---33---32---31---30
3.91 teeth 43--42/41-40---38---37---36---35---34---33---32
4.10 teeth 45--44/43-42---40---39---37---36---35---34---33
4.30 teeth *----*----44-43/42-41---39---38---37---36---35
4.56 teeth *----*----*----45--43--42/41--40-- 38---37---36