I heard about a car transmission that included a compressed air capability to aid in getting the car moving initially, accelerating, etc., saving gas. This story was around about the same time that hybrid cars were first being developed.
The story was the transmission would only add an additional $600 to the price of a car, it added only a little more weight to the car, the energy recaptured while braking or decelerating was around 70% as opposed to 30% for electric cars, and it dramatically improved gas mileage especially in the city.
What ever happened to that idea? I can't find anything on it now.