The aeronautical guru is correct, but dont forget about the mass of the tire and the prospective locations. A tire on the roof raises your center of gravity, and your handling suffers a little. Plus the aerodynamics. A tire mounted in the back will lower your COG, and improve your aerodynamics while driving down the road. But just like on the roof, the tire in the rear has negative effects also. Instead of a higher COG, you are increasing the mass behind the rear axle which will lead to more "pull around" when handling although it will also increase downward force due to gravity. Probably the same effect as a hitch mount receiver, then double it for the tire. This will also affect braking very slightly. When braking hard, less weight will tend to shift forward, which will cause the front to dive less and maintain a better stance when maneuvering and braking. This is a plus, but minor. Also the departure angle from obstacles is a factor.

All these forces are minimal, but will help you choose the right location for the right situation. 4wheel on the roof, cruise on the roof, high speed driving on the back, road trip on the back!