When things get really, really funky, sometimes your best bet is to see if you can get a replacement (possibly temporary) and then take apart the strange one to see what is going on.

They're cheap enough on ebay that it might be worth it (just the speedo/odo combo, mind you, not the whole cluster). Or maybe check into a local junkyard. Or... pull out the speedo for a while & take it apart. My only guess is that if your trip is correct, and the other one is fast.... well, it could be a compound issue.

It seems unlikely that one would be fast, and the other right. More likely, is that they are both fast, but one has some portion broken (a tooth on the tripodometer side, say) that just happens to make it closer to right than the whole system being fast.

If you can, I'd really see if you can track down another speedo (junkyard, ebay, unsuspecting neighbors car...) and see how it functions.

Edit: I re-read the post, and think that the possiblity i put out there isn't too likely. but I think trying to pickup another speedo to test would be best course of action.

 

Thanks a lot for your reply, I almost had resigned myself to wait until I would have the time and patience to replace the whole set of gears again and check carefully for misalignments and also for the integrity of the tooth wheel on the shaft and the two on the counters. And then, I'm seeing that little by little the two counters are slowing down. By now, the overall counter is still a bit too fast but has almost become the right one, whereas the trip counter is now way too slow. I have reset it multiple times to see if that would make a difference (by the way, I replaced the reset button, installing a variety that is much more robust at the angle). So, while the counters are accruing miles at different speeds, at least now the problems seems to be pointing at only one counter being 30-40% slower than expected. And this is more "understandable" as it likely implies than the load is too strong (I think the trip counter has a shaft connected to it on the back which is used for triggering the service light?) on one of them and it makes it skip teeth at a certain rhythm. Much easier to understand than trying to explain how one could be going much faster than expected!!!!

Xavier