There's a possibility that the trip odometer finger has just gotten broken or jammed in the reset mechanism. That can happen. Unfortunately, you probably need a new speedometer drive gear.
You have a few options.
1) find a working instrument cluster, and steal the gear from it, or swap the whole unit, or just the speedometer head.

2) buy just the gear from Kaplan's web site, as given in the previous post. $29 delivered. Not too many other do-it-yourself options.

3) Send the unit to Overseas Speedometer for a rebuild. In which case, he will install the new gear for you and send it back. You'll be without for a few days. As with the other options, you get to do the removal, installation yourself.

4) Take it to a shop and they'll do this repair for about $300 or even worse order a speedo head from Volvo for $400 and install it for $200.

You can remove the cluster yourself, remove the circuit board and the speedo, and take the silver drive motor out of the side to confirm that yes, the gear is broken. It's a tan plastic gear less than 1/2 inch diameter- it usually loses 3 or 4 teeth and jams right up. It would be best to take care of it before the mileage gets too far off.

Good luck!
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Rob Bareiss, New London CT ::: "Roterande Fläkt Och Drivremmar!"