First, you need to figure out what percentage the MPG reading is off by.
1. Figure out the percentage using this formula: GT average economy minus calculated economy divided by the calculated economy.
2. On the tuner go to Main Menu->Vehicle set up/settings->Adjust economy display.
3. Increase, or decrease by whatever percentage you are off by.
*Increase the percentage if the GT is reading lower mileage than you are getting. Decrease the percentage (go negative) if the GT is showing higher mileage than you are getting.
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