When changing wheel sizes, final drive ratios or Speedo clusters it is essential to make sure the speedo cable worm and drives are correct for the car. There are many combinations available, some work better than others, but one which is suitable for the application can usually be found.
This is for a single pinion drive with 16 teeth.
These are now very difficult to source, new or used.
Guessworks custom makes these in house, and as such is the only manufacturer of these pinions.