After you make tire and/or final drive changes you need to change the internal reduction ratio of the spedometer drive within the transmision, either by changing gears or in the case of modern transmissions with an electronic speed sensor by changing the output of that sensor.

If you start playing with final drive gearing and tire sizes I think you'll notice something that's counterintuitive but true. The RPM you're turning at a given speed indicated by the speedometer won't change but your true speed vs. what's indicated by the speedometer will.

Think of it this way.

The tachometer measures engine RPM.
The speedometer measures driveline RPM after some gear reduction within the transmission but before the final drive, and displays it as a MPH display. They are both measuring RPM but the speedo is calibrated to display MPH for a given RPM reading.

The above is not applicable if the vehicle uses a wheel speed sensor to determine speed though I don't know of any that do...