I was listening to the ABC Saturday morning show and there was this bloke on talking about having a solar panel on his car.
This feeds power into his cars electrical system and compensates the draw of the electrical components have on the electrical system. He says that a cars battery is only about 70 to 80% full and gets belted around by all the new fandangled stuff that are in cars now..ie, elec windows, elec dash and ECM systems, so this drags the engine alternator a bit to compensate for the elec drag.
Making a short story long, he averages about 25% better fuel economy, he did'nt say at what speed he travels or driving techniques he uses.
But anyway, the best fuel economy gadgit is your right foot on the little peddal.