The term that actually applies is HEV, which stands for Hybrid Electric Vehicle. These are the most fuel efficient vehicles available to most of us.
While the actual configuration may vary due to manufacturer design intentions, they typically combine a small (by regular vehicle standards) gasoline engine with an electric motor.
While the gasoline engine does most of the locomotion at highway speeds, when less fuel is required, it is also on hand to provide the extra power that the electric motor cannot provide or sustain for long.
The battery/electric motor combination can get the car moving in some cases, maintain systems without the gas engine having to idle at stops like lights and on the road, and can help the gas engine as it drive the hybrid vehicle down the road. Not only is fuel saved as the electric motor keeps the vehicle’s systems at what would normally be idling situations, but this means less pollution is released into the air, typically within the confines of our most polluted areas… our cities. .
While the vehicle is rolling, particularly in situations of inertia, the battery that provides energy to the electric motor is recharged. When braking at lower speeds (inside town, stop-and-go), regenerative braking is typically used to stop the car rather than the brakes themselves. In regenerative braking, the electric motor actually changes polarity, becoming an electrical generator that recharges the battery. This switch from motor to generator creates reverse torque that is used to slow and stop the vehicle. At highway speeds, normal braking occurs.
Sometimes the charge inside the battery can drop below the necessary level and at such times the small petrol engine automatically turns on to recharge the battery.
However, even with hybrids, there are different types of vehicles to meet consumer needs. There are hybrid SUVs, trucks, and small to midsize sedans like the Toyota Prius, which is one of the most fuel efficient hybrid vehicles available and gets an average of 50 MPG. Larger hybrid vehicles, such as SUVs and trucks, tend to provide fuel economy in a relative sense. The fuel economy of these larger hybrid vehicles tends to be roughly equal to that of a gasoline sedan.