You can do that, but we have the law of conservation of energy to deal with. So, when you do it, you slow down the car faster by applying a decellerating force on the car. Mechanical advantage, while very useful, doesn't circumvent the law of conservation of energy. It only allows you to make better use of the energy you have.
So previously we were talking about regenerative energy which comes from breaking and downhill runs. In those cases wasted energy can be utilized to give better overall efficiency. This works because we are identifying the energy sources as kinetic energy of the car motion, or potential energy of the car as it goes from higher elevation to lower elevation. The old ways tend to waste this energy (unless you drive in a very smart, and perhaps dangerous, way), while the new systems can recapture some of this wasted energy.