Thanks for the comment's guys...I am aware of the small & large size drive gears. Its stock 3, Saginaw, speed and 3:73 gears. The larger tires require a driven gear adjustment. I went from 22 (original) to 21 (red) driven gear but it needs a 20 tooth. Speedo is reading about 4mph slower than actual using GPS app on my phone.
I have dealt with this on other cars I have. I assumed my drive gear is the 1.84 8 tooth which is why I asked the ? on what a stock Saginaw came with with a truck with 3:73 gears
When I put the 20 tooth in the speedo would intermittently jump up some but not read nothing consistent. To me it seems like the teeth are just missing the drive gear. Its a GM driven gear.
This link (below) is what I used to determine what tooth gear I need, which said 21....although actual is a few mph off. I will measure tire diameter to confirm if it is what tire spec sheet says...in the past those are not accurate.
http://www.stl-vettes.com/65Vette/GearCalc.htm
I also have one of those right angle speedo gear reducers...it is not what I need for this