Joe,
I founnd this info on the web. If you have the blue driven gear, that would have 20 teeth. From your earlier post you need to cut the speed by 20%, so you would need to go to 24 tooth driven gear to achieve that. From this data, I don't think you can do that with out changing the drive gear. That can only be done by removing it from the shaft inside the rear housing.
You could try to call someone like Hanlon Motorsports:
http://www.hanlonmotorsports.com/or theses guys:
http://www.5speeds.com/ to see if they make other driven gears.
Here is the info:
SPEEDOMETER DRIVEN GEAR
New OEM AC Delco gears are used in Muncie, GM T10, GM Super T10, Saginaw 3 & 4 speed, GM T5, Powerglide, 200 & 350 transmissions.
18 tooth brown dimension -1.75 x .305 x .810
19 tooth white dimension -1.75 x .305 x .810
20 tooth blue dimension -1.75 x .305 x .810
21 tooth red dimension -1.75 x .305 x .810
22 tooth grey dimension -1.75 x .305 x .810
18-22T .810 diameter gears must be used with a 1.84 OD speedo drive gear.
22 tooth green dimension -1.75 x .305 x .870
23 tooth black dimension -1.75 x .305 x .870
24 tooth yellow dimension -1.75 x .305 x .870
25 tooth orange dimension -1.75 x .305 x .870
22-25T .870 diameter gears must be used with a 1.76 OD speedo drive gear.
I have a grey 22 tooth gear in a spare transmission in the basement, I could mail it to you if you want to see what a 10% reduction would get you.
