I actually did a web search and found that someone had a patent on the invention for a roof mounted generator back in 1999! I'll try to post a hot link to click on it.

Propeller wind charging system for ... - Google Patents


I believe they got a 40 percent charge rate on their home made vehicle, so I assume the vehicle would not be able to run on the fan generator for an indefinite time (would stll need to be plugged into a charger)

Edit: A wind powered generator that would be powerful enough to keep the electric car's batteries nearly fully charged, at normal driving speeds would basically be a car that used NO fuel and would appeal to anyone that could afford its high price. One can only dream!