The process of converting energy stored in a battery to kinetic energy with the help of an electric motor is more efficient than converting energy stored in fuel to kinetic energy using internal combustion engine. This is because for internal combustion engines, a lot of energy is wasted as heat.

Hybrids have better fuel efficiency for both city and highway driving but that of city driving is more. The difference is not much for highway driving when compared with an internal combustion engine car of similar engine capacity.

Now, let's see how better mileage is achieved for both city and highway driving.

How hybrid cars achieve better mileage for city driving.

The main means through which hybrid cars conserve fuel for city driving is the use of its high voltage battery. The battery is charged by the internal combustion engine and regenerative braking. Kinetic energy of the ICU is used to power the generator sets that converts the kinetic energy to electrical energy used in charging the battery. For regenerative braking, the kinetic energy lost during braking is converted to electrical energy through the use of generator. Note that it is same generator that acts as motor to drive the wheels when battery is being used to power the car. The hybrid car alternates between the battery and ICU in traffic and at low speeds thereby conserving the fuel the ICU would have consumed while operating continuously.

How hybrid cars achieve better mileage for highway driving.

At highway speeds, the fuel saving is not as much as that of city driving. However, the Atkinson cycle engine that the hybrids use is more efficient at top speeds than traditional internal combustion engines. So, a reasonable amount of fuel is saved.

Another way fuel is conserved on the highway is through regenerative braking. As said ealier, the energy that would have been wasted when only pads and discs are used for braking is converted to electrical energy which charges the high voltage battery (traction battery).

Similarly, during deceleration, the battery is also charged as the kinetic energy at the wheels is converted to electrical energy that charges the battery.