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Every so often the speedo starts making a clicking noise and then starts reading excessively high speeds. RPM indications, vehicle capability and the real world make it clear that I'm not really doing 240km/hr. What's going on and how would I look at fixing it?

A visit to an auto electrician is in order to at least get an idea of what may be wrong. It may be something reasonably easy to fix but then again it may not.

Certainly not a common problem that we are aware of.