While it's true that liquid fuel is more energy dense than batteries, you have to remember that most Internal Combustion Engines are only around 20% efficient.

While it's true that liquid fuel is more energy dense than batteries, you have to remember that most Internal Combustion Engines are only around 20% efficient. So even though you're carrying around a huge amount of potential energy in the form of gasoline, your engine can only covert about 20% of that into motion.