ok, i was trying to avoid too much info; but here goes:
last year when i had a 4sp saginaw, i bought a 17 tooth gear (to replace a 21 tooth gear) to increase the speedo reading. funny thing is, it DECREASED the speedo reading by maybe 5% (without bouncing or jumping, very smooth). this 17t gear was a brand new sealed GM part. i never solved that problem because the saginaw broke soon after that. then came the M20. anyway, my 3.31 10-bolt broke last month, so i replaced it with a 2.56 12-bolt. (please don't yell at me, i'm paying the consequences now--no launch)
anyway, the current driven gear has 21 teeth (red). i was too dumb to count the teeth on the drive gear when we rebuilt the M20, but i believe it was white. i know the color code for driven gears, but none of the 20+ web pages i've been to list the colors of the drive gear.
i need a 29% increase in the rotational speed of the speedo cable, so i need like a 16 tooth gear (+31%). since i already had a 17t gear with less than a mile on it, i figgered i'd see if a 17t gear (+24%) would satisfy me. well it did the exact same thing that happened with the saginaw! now i doubt the drive gear in both tranys have the exact same defect, and the 17t gear was brand new (GM, sealed when i got it).
why do i want to use an external adapter? well, i don't want to change the drive gear because:
1. i've been inside that M20 before and i'd rather not go through that again.
2. if i don't like the 2.56 and change it back to 3.31, i'll have to swap the drive gear again (see reason #1).
unless a 'speedo gear reducer' simulates a reduced number of teeth on the driven gear, then i don't need a reducer. what i need is an adapter that will increase the rotational speed of the speedo cable.
so. the question is: does anyone know of a 'speed up' speedo gear adapter? are these still available? how much and where can i get such a thing?