Imagine running your car off of wind power. Like the sailing ships of yore that circled the Earth without using a drop of fuel, you would be able to go forever and spend nothing. Perfect freedom! Except of course that you are then forced to wait for the wind. While a sailing car may not be practical for everyday use it is an interesting engineering challenge to build one.
The Greenbird is an example of a wind powered (sail powered) racing vehicle. Looking at the picture, you can see that the "sail" resembles an airplane wing sticking up vertically. The rest of the craft is basically a needle with a blister at the back just large enough to hold the body of the pilot. There are outriggers on each side, necessary to keep the sail from simply toppling over sideways. Looking at the picture of the man seated on the outrigger gives a good idea of the scale. Like these record setting gas sippers the Greenbird is too small to carry cargo or passengers. Its small size is necessary to reduce the aerodynamic profile and increase the top speed. After all, the goal is to set a speed record, not bring home the groceries. It is made of carbon composites so it can be light yet strong.
It was designed to set a new world land sail powered speed record, which it successfully did. It reached a top speed of 126.2 miles per hour. The record was set on March 26th 2009, a high wind day. The top speed of the Greenbird can be 3 to 5 times faster than the wind speed. This depends on resistance coming from the ground. Smooth, hard surfaces are better than rough or loose packed ones. To understand how it can reach top speeds greater than the wind speeds, you have to take into account the apparent wind.
Although a wind powered car would not be practical for local trips around the neighborhood or city driving, we can imagine a future where they have a role. In the current world, gas mileage is not the most important factor. As a result we use the same vehicle for everything. In the future when gas mileage is more important, maybe even the most important factor, I think we will begin to use different types of vehicles for different purposes, choosing the most fuel efficient for each. For example, local trips could be taken in a personal Neighborhood Electrical Vehicle or NEV. Long haul trucking could be done by giant sail powered trucks, something like the sailing cargo vessels of old. Special highways or lanes could be set aside for them, and they could glide along at constant speeds set by the wind. Most long haul freight can be done without strict time limits, so the sail trucks could simply use whatever wind there was. Sail truck highways could be built along the windiest routes.
The picture to the side is an imaginary representation of the idea of a sail truck. A real sail truck would likely have the same general characteristics. A long, light needle shaped body with one or two airplane wing like sails on the back. The most important feature missing is that a real sail truck would need some kind of outriggers to keep it from being blown over by a crosswind. Another option would be to have a low, wide body which fills in the area covered by the outriggers with cargo space. That would have greater air drag and thus a lower top speed and lower threshold wind necessary to move at all but correspondingly greater cargo capacity.
Another option is to build long outrigger free needle like bodies and secure them to the highway with a special kind of runner. This would replace the keel of a sailboat with an attachment to the ground. It would of course require specially built highways. The highway could have a deep groove in the center of each lane and the sail trucks could have a "keel" which descended into the groove. The keel would have something like wheels which ran on low friction rails set into the side walls of the groove. This would require a bigger investment in highway infrastructure but would permit more aerodynamic sail trucks that could thus reach faster top speeds and operate at lower threshold wind speeds.
I think this kind of innovation will be necessary to keep us moving cheaply in a future where oil and gas prices will be very high and rising. Better to start early, because the sooner we start the sooner we save on gas!