A/F gauge hookup..

[LightningGSX]

In this case, ground is also a 0 volt reference for the gauge.And its not uncommon to see a 1/2 volt or so difference in ground potential between different grounding points in a car.The 02 sensor puts out 0-1 volts relative to the ground potential of whereever its grounded at.So if you ground the gauge somewhere with a different ground potential, you'd have to either add or subtract the voltage drop from the gauge reading.When you're dealing with a rather small 0-1v signal, 1/2 v is a huge inaccuracy.