A hybrid system can make a car more efficient. And efficient car mean the same performance on less fuel, or more performance on the same fuel. Lexus pioneered the second route. Now BMW has taken the idea and gone ever so slightly berserk. BMW ActiveHybrid 7 is a two-and-a-bit tonne luxo-barge that gets to 60mph in 4.7 seconds.
When you drive it like that, the hybrid system isn't doing much. It's only a mild hybrid, which means the electric motor, sandwiched between the engine and transmission and turning always at engine speed, is relatively small and incapable of driving the car unaided. Instead, what it does is fill the gaps when the V8 isn't working very efficiently, so when you're driving gently you're using less throttle. | 
