A hybrid car is inherently more efficient on the highway than a traditional car. Hybrid cars get more energy from a gasoline engine by always operating at the peak efficiency. There is a certain RPM and load that will use the least fuel per Joule of energy extracted from the engine, and that sets the conditions under which the engine runs. Combining this with a design of the car geared toward overall economy of fuel, and that's why hybrids do better.
The tradeoff is the inefficiencies of the generator, batteries, and electric motor, which are all extra elements in the power train. Theoretically, it is possible to operate the same gasoline engine with the same efficient body design at the correct speed and gear ratio that the traditional configuration would end up being more efficient. Of course, starting, stopping, and varying highway conditions (hills) prevent such ideal operation, hence the hybridization. By storing the energy from an efficient burn and using it to drive a motor (which doesn't much care about the car's speed or acceleration), that's how the hybrid gets its economy.
What I'm trying to say is that hybrids use the same internal combustion engine technology as all cars do. They just harness the output better. We still eventually have to get away from burning petrolium to drive ourselves around.