While driving to work yesterday, I looked down to see the speedo say I was doing 62mph. As I was in a 30 and in a line of cars, I thought it was strange. When I stopped to give way at a roundabout, it stayed at 62. I did google the issue, but everyones posts/threads seem to be the speedo stuck at zero. Is this to do with the speed sensor on the gearbox? The steering, is not heavy and seems to adjust its self between slow and fast driving.

Is the speedo converted using a chip  on the back of the clocks ? I always start with these and bridge them out by re joining the signal wires,Usually a green and yellow.Try this and see if the issue stops.If it does you have found the problem.The box sender units are pretty reliable in all honesty.The clocks are not so but start with the converter chip first.

The car is still in KPH, but I have MPH clocks in it, I have a converter that I haven't fitted yet. From memory I don't think the car has ever been converted and the loom is intact, but I'll need to confirm that. My two week holiday is starting at the end of this week, so I'll have a few "zed fixing" days I think!