...Only thing that alters the reading of the speedo is your tyre and alloy size!...
On older VW's with 020 'boxes, the speedo drive gear had to be matched to the final drive ratio fitted in the 'box. Different speedo drive gears were available with different numbers of teeth and, rather helpfully, used different coloured plastic, useful when you don't have the original so have to buy a few and swap them in and out until you figure out which one is the right one. Been there done that on a Scirocco I used to own.
I know that nowadays they use an sender with an electrical connection to the back of it rather than the mechanical drive of old, but the same principle of matching the speedo drive to the 'box fitted applies. So long as they're not messed with, it ought to read correctly.