Here is a quick fix for the common speedo problem of the shaft for the gear reducer rounding off in the tranny tailshaft housing.
If you remove the grear reducer from the tranny, you find a square drive shaft aprox 1/8" inch square and 2 inches long, this is the "male" end of the drive assymbly for the gear reducer and tends to get rounded off about half way down it.
Take a flat washer and carefull grind it down enough to drop into the threaded coupler around this shaft. When reassembled, this acts as a shim and moves the shaft "up" enough that it is square again where it sets in the drive gear.
I'll try and get a few pics of this posted in the next day or so..

 

Cheap and quick. I like it - I always appreciate a "tightwad fix" that works.