The easiest way is to work out the % difference between the 2 wheel sizes and then apply this to the speedo reading (as the % change in dia = % change in circumference, assuming the tyre side wall dimension remains the same).
So, for example, if you were to change from a 16″ to a 17″ wheel that is a 6% (5.88% if you want to be super accurate)increase. Therefore, your speedo would then read 6% slow. So at an indicated 100mph you would be traveling at 106mph.
I’m not sure how you would recalibrate the speedo in practice though?
The best way would be when changing wheel sizes also change the speedo drive or buy one of those electronic bike type speedos that can be reprogrammed.
hope this helps
Katana