dluck: even at 45 mph, probably half or more of the power required to go down the road is lost to aero drag.
Have a look at this tool:
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
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Also the boattail length/curvature will create a sweet spot for a particular speed.
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Cd is effectively constant for our purposes - the shape either works, or it doesn't, regardless of speed.