[MetroMPG]

That's right: a calibrated MPGuino will properly report MPG when in lean burn. The ScanGauge won't. (Note: the MPGuino is not a commercial, or store-bought gauge - it's a DIY or custom made gauge from forum members here.)

Lean burn is the secret weapon to getting great MPG in your car, provided you mostly do the type of driving where it's in play (steady cruising at moderate speeds rather than city type driving).