Here is another concept for vehicle propulsion: hydraulic power. Valentin Technologies is developing the Ingocar which it hopes will achieve a gas mileage of 170 MPG or only 0.590 GPHM (gallons per hundred miles). The vehicle is light and aerodynamically streamlined, but the real secret is in its hydraulic drive system.
The idea is that a normal gasoline burning combustion engine is used not to provide propulsion, but to pressurize a reservoir of hydraulic fluid. Each wheel of the car will then have its own hydraulic drive which will convert that pressure into motive revolution of the wheel. This system can run in reverse during braking. The wheel drive motors act as pumps, converting the kinetic energy of the car back into reservoir pressure.
Whether this technology will prove out or not is unknown, but it is good to see that we have many options under development. We are going to sorely need a way to drastically increase our fuel economy in the decades ahead as the global oil supply becomes ever more expensive. Each idea under development gives us one more arrow in the quiver for the hunt to save on gas.