This is how it was designed by VW and what is actually built in into the cluster's software. Also, the coolant temperature gauge is even more inaccurate, if you ask.
This is NOT something what can be corrected with VCDS and/or tyre sizes.
There is a fix for both, though, the inaccurate speedo and inaccurate coolant temperature. But... It will require very expensive equipment in order to dump the EEPROM, do the required amendments and push it back, leaving alone the entire operation is very risky; you might end up with bricked cluster which the oly way to remedy is to throw into the trash bin.
Is it worth?