My non-hybrid Civics have always displayed much the same behavior, although the drop-off is more like 5% than 10%. Part of it here in the Denver metro area is that oxygenated fuel (all ethanol additives now that MTBE is gone) is required during the winter months. At least some of the gas stations put up signs in the spring when they receive their first load of ethanol-free gasoline. What effect does cold weather have on the electric side of the hybrid power train?

Ditto the Honda Insight. Honda dealer 'mileage specialist' says it is because of the winter fuel formulation.