i've been thinking about this same issue as to generating electricity or power in a vehicle just by using the air friction forced upon it by the increase in the vehicles velocity. I was thinking about designing the vehicle so that the front captures the air and channels it into a funnel such as your describing and turns the turbines. So in theory the faster you go the more energy is produced, but with a very efficient turbine i think it would work out great. What do wind turbings turn at? i believe they only require about 5mph wind and they really start generating at around 10-20 mph range