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.
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