The vast majority of vehicles on U.S. roadways in the year 2040 will still use gasoline, according to a projection from the U.S. Department of Energy.

A whopping 78% of light-duty vehicles will still be sold with gasoline engines in that year compared with just 1% plug-in hybrids, 1% full-electric vehicles, and 5% gasoline-electric hybrids, according to the U.S. Energy Information Administration's (EIA) Annual Energy Outlook 2014 (AEO2014).

The report predicts an increase in "micro-hybrids" with stop-start, regenerative braking, and other technology that improves fuel economy for vehicles with gasoline engines.

Additionally, the report anticipates the reduced market penetration of flex-fuel vehicles, which are neccessary to meet the renewable fuels standard. The phase-out of corporate average fuel economy (CAFE) credits for their sale and limited consumer demand is slowing their adoption.

The energy outlook predicted that fuel efficiency of the nation's cars and trucks will rise to 37.2 mpg by 2040 compared to 21.5 mpg in 2012.