Originally Posted by Traveler (Post 5474566)

A mild hybrid is usually a car where the Internal combustion engine is assisted by a small electric motor powered by a small battery. It is used only to give an initial push to the car when starting from standstill or from very low speeds.

A Start Stop system is where the engine shuts off when the car is brought to a standstill, like at a red light and then automatically starts when you release the brake in order to move from the red light. The system also works in bumper to bumper traffic and there is also a button to disable this feature.

The two are mutually exclusive and cars can have either both or any one of the systems. It is also possible to have Start Stop without mild hybrid like in Jeep Compass.

Start Stop is touted as a fuel Saving technique and to reduce CO2 emissions as it cuts the engine idling duration.

Mild hybrid is more of a performance feature that gives the car improved initial pickup.