I think I have the same problem. I havn't verified it with GPS yet though.

My cluster was rebuild. I also went from a 2.76 gear to a 3.23. I changed out the speedo gear from a light blue/green(original) to a yellow gear. I don't remember the tooth count off hand but it's the correct one for 3.23.

Dad was following me for the cars first trip out of town since the restoration. I set my foot to 60mph but dad said I was very close to 70mph. I was turning 3000 RPM. The online RPM calculators say I should be turning less then 3000 at 60 with 3.23's and stock F70x14 tires.