>
> I can readily picture that these two gears got gunked or gummed up and
> that
> while it didn't require a great amount of force to move them past the
> sticking point it was almost certainly enough to dislodge that crappy pot
> metal gear from its press on the main shaft after repeated applications.

That may be the case, but if so, the difference in the amount of "dislodging
force" applied by a gunked/stuck tenths wheel on the trip meter to the main
odo gear will not be much between a moving and a stopped vehicle. In the
case of a stuck tenths gear causing the main gear to come loose from the
shaft, you'd have two opposing forces:
 -the reset "fingers"on the cam lobes of the trip odo, to the stuck-on lower
tenths gear, through the tiny idler gear to the upper tenths gear on the
main odo shaft

this opposed by

 -the main gear's desire to NOT rotate due to being connected to the speedo
cable via a worm gear, which as we all know can only drive, cannot be
driven.

Now, the amount of pressure from your finger necessary to break the gear
loose would be the same whether that worm gear is moving or stationary, the
only difference being the distance you have to push the lever. There's just
no appreciable difference between pressing it while in motion vs pressing it
while stopped. I'm not saying pressing didn't pop the gear loose; just that
it was bound to happen either way.

-- 
John Bange
'90 Vanagon - "Geldsauger"