Surely lean burn has something to do with it but I don't think that it is the biggest factor. The engine is actually completely different in the Insight CVT vs the MT. According to InsightCentral the CVT's compression ratio is 10.3 to 1 while the MT is at 10.8 to 1. The higher compression the better FE wise. I'm pretty sure that the MT is geared up quite a bit taller as well.

The MT's lead is seriously diminished with the HCH but the lead still goes to the MT if only by a hair according to our database. Summer is not giving the gen 2 HCH as big of a boost as I expected. I don't think the gen 2 is going to catch the Prius or the gen 1 MT.