Such a device would produce less energy than the extra fuel it would require to turn the fan.

Just reduce the coefficient of drag of the car and it would use less energy. Remember that energy saved is like energy generated, only more efficient.

I agree with the other two posters that you are better off reducing drag. The old penny saved is a penny earned addage applies here. If you are looking to recapture energy expended by the the vehicle, look to use the heat from the engine to turn a steam turbine. The heat produced by the internal combustion engine is pure waste and you won't likely be penalized as severely as you would with the wind turbine, if at all. BMW has done this with some success.

Hyenuf is correct. If you were to, say, mount a windmill in the back of a pickup truck, you could capture wind energy as you drove down the road. However, your car's engine would have to work harder to overcome the drag produced by the windmill's wind resistance.

Even if you feed the power you generate from the windmill back into the car somehow (let's say it's an electric truck), you would have transmission losses due to heat (resistance) in your wires, the less than 100% conversion rate of your windmill (vs its wind resistance), and the normal inefficiency losses that your vehicle naturally has.

If you leave that all aside though, and assume for the sake of argument that you could convert 100% of the wind energy with no extra wind resistance and no transmission losses, the best you could possibly get would be a wash in terms of energy produced vs energy spent.

There, I said in 3 paragraphs what Hyenuf said in one sentence.

A person could make a more efficent cone shapped fan blade that would reduce to wind drag and use a powder based lubracant to solve some of the problems and instead of trying to use the power generated on the car use it to charge batterys to power home needs light all lighting for each night or even computers anything that can run with 12 V power... just a thought

what if...

1. someone placed a solar panel on the roof of the car (as large as a conventional roof)...

2. added generators to the braking system...so that when the person lets off the gas or brakes, the generators kick in...

3. added small "turbine style" wind generators on the front of the car and in the rear where the air usually flows back toward the car or in a downward motion...

4. and utilize an electric pulse system vs. a standard (always on or off system) to run the electric motor...?

Look here for some information on thermocouples and recovering automobile heat. It is not a very efficient process, but answers some of your questions. http://tinyurl.com/5nw2h7