First, you will need to figure out how far off the speed is in percentage.
1. Figure out the percentage using this formula: GT speed displayed minus Actual vehicle speed, divided by Actual vehicle speed.
2. On the tuner, go to the Main menu->Vehicle settings/Vehicle setup->Adjust speed display.
3. Adjust the speed displayed on the Device up, or down by whatever percentage it is off.
4. To save, press Go Back.
If it is still off, you might have to do the process over again.