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As you face the back of the gauge, there are two calibration points. Lower left is adjusted with the ignition off to find true "zero"-"E" on a fuel gauge or "L" on a coolant temperature or oil pressure gauge. Upper right is adjusted with the ignition on with five volts of power to the gauge. This is where you adjust the needle for actual conditions. For example, "F" for a full tank or normal range with engine operating temperature at approximately 180-200 degrees F. Same is true for oil pressure.