Yeah, sounds like a fun project. I don't have a hood ornament, but a magnetic antenna mount would help me make one
I read that there are ways to calculate air resistance of a vehicle, but it appears to require a lot of knowledge of fluid dynamics. By doing some math with groundspeed vs airspeed (and perhaps an accelerometer or GPS to take road grade into account) the vehicle's wind resistance would come out as a number? ... good information on which to base further experimentation on drag-reducing mods to the vehicle.
But if you really want to improve vehicle mileage, I suggest only travelling in the direction of the wind
