Maybe buy a dial converter? I think you can get adhesive dial faces which are re-calibrated to correct wrong ratio/readings. I guess the dial you fitted didn't come from the same car, and/ or else your diff ratio is also different.

 

90% of all cars leave the factory with speedometers reading 10% lower than what your actually doing, hence the law giving you a 10% buffer for speeding tickets, unless it's miles out I wouldn't worry

 

Chances are your car has a 4.3:1 diff whereas the 6 speed gearbox came with a 3.909:1. As your speedo drive is taken from the gearbox and is therefore unaware of this, it's working on the assumption the diff will be reducing the rotational speed of the rear wheels rather less than is now actually the case, thereby giving you a high reading.

Easiest solution maybe to get some custom dials made up, graduated to suit, or source a 3.909 diff.

Edit: Further searching suggests the 6 speed gearbox was also fitted in combination with a 3.636 final drive. This would make the situation even more noticeable - which model did your 6 speed originate from?

Check your new dials against the originals too, UK and JDM dials are graduated slightly differently as well (on the MK1 at least).

 

I'm sure there's a thread on here somewhere telling you which speed sensor you need, just swap it out with the right one

 

Swap speedo gear with your old gearbox Job done..

6 speed compared to 5 speed is pretty much the same in relation to speed.. Just your get there quicker due to the extra gear.