The general rule of thumb in aero is this: Split the air quickly, and then bring it back together gradually and slowly. An ideal shape would be an infinitely long teardrop. The reasons for bringing the air back together slowly are, not disturbing the laminar flow over the surface, and not creating low pressure regions that in turn enact drag on the surface. Of course in ground vehicles other things come into play like the ground effect and wheels. Believe it or not, uncovered spinning wheels wreak havoc with aerodynamic efficiency.
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The general rule of thumb in aero is this: Split the air quickly, and then bring it back together gradually and slowly. An ideal shape would be an infinitely long teardrop. The reasons for bringing the air back together slowly are, not disturbing the laminar flow over the surface, and not creating low pressure regions that in turn enact drag on the surface. Of course in ground vehicles other things come into play like the ground effect and wheels. Believe it or not, uncovered spinning wheels wreak havoc with aerodynamic efficiency.