[MetroMPG]

Remember that neither the ScanGauge nor the UltraGauge will be accurate with a lean-burn engine. They monitor air consumption and calculate fuel use based on an assumed stoichiometric A/F ratio. They don't detect or know about lean burn in performing those calcs.

MPGuino is the way to go.

darcane's description is what I would have said. Very slight stumble, throttle response "deadens" while in lean burn (not nearly as responsive). In the Insight it's obvious when you enter lean burn because the instant MPG display jumps up by ~25 MPG.