Good question. I tend to watch the average more than the instant (but that will be changing now that I've got my
SG2 and I can program the Trip AVG value to show as one of the gauges.
Anyway to your question...
With the engine idling, and the car in neutral, the average mpg shown on the
ScanGauge in a glide from 90-70 km/h was 550 mpg. That's the highest I've made a point of recording.
What's yours?