It sounds like your car has a Speedo converter as your title suggests.. Can you take a picture of the top of the gearbox. There is a speedo cable that screws into it you should see it, It probably goes into a metal box, then the box screws into the gearbox! If you do have one, removing the speedo converter and screwing the speedo cable directly into the gearbox will make the dials read correct. I have NO speedo converter and my dials work fine! - That's my guess anyway!