Take this with a huge grain of salt, as it's been some time and I'm posting from the office....
I put a different speedometer in one of my "bumpside" Ford trucks a couple years ago, and just out of curiosity I dug into what makes these things tick (w/the old odometer). Upon disassmbly I seem to recall that each "9" digit had a small tab
inside the number that would "catch" the digit to the left to make it move/cycle to the next number. If you can source a junkyard/used odometer that you don't care about, you could disassemble it, and perhaps you'll see something similar.
Of course this begs the question: Why is it doing this? Is this a new (to you) odometer, or a new car? Or has this just started happening on a known car?
Another thought, if you don't care about the mileage: Maybe spin the "ones" digit slowly to "9" and see if it make the "tens" number turn-over as it rotates to "0". Why? Based on my questions above, if this is a new odometer,
maybe it's been damaged/tampered with.
But that's just a guess.