I have experienced that same effect just today. Was traveling in areas that had 20-30 mph speed zones and I used the E mode 75% of the travel time until finally the battery got to low and my analog mpg gauge was stuck at 10-20 mpg for some time there after. Total trip mpg was well below 30 mpg, so I agree with your theory.

Orcrone - is your commute level or varied terrain?