I had exactly the same problem. Speedo went nuts, the Odometer was fine. These guys fixed it for me. I compared it to the GPS I've been using as a speedo. Every five mph from 25 to 80 mph it was within 1% the whole way. They reset the odometer and fixed the speedo for $85 not counting shipping. They said it was an cheap old Japanese designed speedo. but they fixed it better than new.

http://www.foreignspeedoinc.com/

We all like to think of ourselves as rugged individualists. But when push comes to shove most of us are sheep who do what we are told. Worst of all, a lot of us become unpaid agents of whoever is controlling the agenda by enforcing the current dogma on the few rugged individualists who actually exist.