[james]

"Lean burn can only be maintained for a few minutes at time but if you don't move your foot at all, it will come back after a few seconds of running rich."

What you see there is something called a "NOx purge". As I understand it, lean burn actually creates quite a bit of nitrogen oxides (bad for emissions). The Insight's catalytic converter actually collects these, and when the computer decides that the collector is full, momentarily enrichens the mixture to burn them off.

 

[james]

I see more like 2 minutes between lean burn cycles. You can actually feel them if you're paying close attention: there's a little bit of a surge, as though you'd stepped on the gas.

I see it more around 65-75 (since I don't do much 55 mph driving, except up hills). Given a slight downgrade, such as on the down side of a freeway overpass, I'll see the mpg go to around 90-110, vs around 75 "normal", or 50 in purge.