Aftermarket speedometer

Real world testing... knowing the current gear number of teeth you can drive at a speed, and see what your speedo reads, at the same time what does a GPS read. The speedo and the GPS will make a ratio and that will be the number to use to calculate the new gear teeth. Might be smaller or it might be greater number if teeth.

To make speedo read faster fewer teeth then you have now
To make speedo read slower more teeth than you have now

There are many threads on FABO some even have equations.