Dave,

When the pack gets low enough it stops depleting? No, that is when it is too low!! Below 2 bars of battery pack charge, auto stop no longer works, and you no longer get assist, but the battery pack still depletes, especially in forced auto stop mode. I have often depleted it until there are no, zero, battery charge bars showing, nada. And in glide mode with the engine off, the 12 volt is on its own running the radio, fan, instruments, and at night, the lights, and it is not being supported by the pack. When the 12 volt gets too low it does not have even enough power to engage the IMA and battery pack, let alone start the ICE. At that point all you can do is get out the jumper cables. I know, I have been there many times, testing the limits of the HCH.

As far as lean burn is concerned, despite all of my experiences in the HCH, I don't really know when the HCH goes into lean burn mode. I do think, however, that I am in lean burn above 30mph, accelerating in 4th gear until 42mph, then 5th gear (manual transmission) when I have green battery charge bars showing. But maybe I don't know what I am talking about.

In any event, whether it is in lean burn or not, P&G in the HCH gets better mpg than in cruise control at any speed. In cruise control mode, I prefer 44mph (55mph on an interstate) for a good balance of mpg, flow of traffic and going fast enough and with enough rpms to deal with the hills. I don't even know if 44mph gets me into lean burn mode, but the FE is better there than any faster.

Bob