The odos fail because the main drive wheel on the digit wheel assembly is cheap pot metal press fitted onto the the drive shaft. It doesn't or at least shouldn't have anything to do with setting the milage while moving. It is possible to disassemble the whole package and drill and tap said drive wheel for a set screw. And once you get that accomplished its best to file a flat spot on the shaft where the set screw makes contact. Its a little tricky but I did it on my '72 911E. One of the major tricks is to get the bezel(?) off the speedo housing without ruining it, since as we all know Porche's solution to spare parts is buy the complete assembly, which I in fact tried to do but was told speedos for that model are no longer available. If you get past that hurdle you have to put the set screw in at angle in order to get to it with the allen wrench after its reassembled. Its very close to the mounting plate that carries the complete assembly.
I think the dead odo syndrome ranks with the timing chain tensioners, rubber clutches, U. S. version distributers, and other poorly designed components that Porsche has foisted upon us. (Remember the first carbs that replaced the early Webers and the '71 911E front suspension?)
And while I'm here, my 911E has taken to hopping like a rabbit, i.e., when I exceed about 70MPH the real wheels start acting like they are out of balance and as I increase the speed it turns into a really hard hammering effect, which sometimes doesn't go away until I slow back down to around 30 or 40. So far I have replaced the tires with Michelin VRs, re-balaced them, and have installed new Knois on all four corners. Needless to say I am not driving my Porsche much lately.
So if anyone has gone through this one I would really appreciate anything that was discovered. I am currently thinking that it may be the CV joints??
Your constructive suggestions are solicited.
Cheers,
McF