Not much really. There's a gear and servo in your transmission that transmits the speed. If the gear broke, you'd read nothing, so I'm guessing there's some issue with the gauge cluster itself. Thankfully, new gauge clusters are cheap. I guess if you have a friend with a bugeye, you could swap clusters (it only takes a couple minutes to remove), and try that. Do the needles move smoothly? Have you tweaked the cluster at all (changed light bulbs, colors, etc)?
Bear in mind that it's normal for the cluster to read 2-3 mph higher than actual. Most manufacturers do this to protect themselves (extra safety factor, if you will), but it sounds like yours is well beyond that.