Back in the late '70s - back before they had sophisticated computers on cars - a couple of guys took a standard big car, and replaced the transmission with an air compressor. They ran the engine only at around 2,300 RPM, the speed that gave the engine the best fuel economy.

The engine ran the compressor to fill a compressed air tank to 5,000 psi. When the air pressure reached 5,000 psi, the engine shut off. It re-started when the pressure dropped to 4,000 psi, always running at optimal efficiency speed of around 2,300 RPM. The whole thing was computer controlled.

The reason that the air pressure in the tank dropped off was, the air was being used to run 4 pneumatic motors - one at each wheel - to power the car. The car ran well, plenty of power, 30% better fuel economy.

This kind of thing has been done for a long time. Many factories that turn out the multitudes of products that we use every day, use pneumatic motors in areas where  larger, electric motors won fit because of limited space. And they have been doing it for a long time, well before the '70s.

This should have been done with cars years ago, and manufactured assembly line style to keep costs down. I wonder why it hasn't been done this way. Do you could think that more money for the fuel companies might be the answer?