Actually, the ones they make for sailboats use a normal automotive alternator, a 12 inch diameter propeller and operate in 10-30mph winds. A 6-8 inch propeller should work fine at highway speeds. I have a propeller if somebody wants to donate an alternator, I could run some tests.
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"The trouble ain't that there is too many fools, but that the lightning ain't distributed right."
Mark Twain