[t_g_farrell]

There is no signal generated, the speedo is an old school analog that has the cable spinning
a magnet around the needle armature which causes the needle to move. Its very precisely
made and can get out of whack by dirt and stuff building up in the speedo, or by well
intentioned lubing of the mechanism or just age.

A bouncing speedo at low speed may be a sticking cable. You can unplug the tranny end
and remove the cable, clean it and very lightly lube and reinsert. If you get really
ambitious while its out unplug the other end from the speedo and clean and blow out the
casing with compressed air and some light solvent. Very rarely do the tranny gears go
bad. Its usually a cable or speedo that causes problems. The cable is easy to address to
rule it out.