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What the graph above does not show is the inefficiencies of converting sunlight into usable energy -- through all the gyrations -- conversion of sunlight, battery efficiency, controller efficiency (they make a lot of heat, which is wasted energy), motor and gearbox efficiency, etc..
True, from a "drag only" perspective, yes, aerodynamics is the big fish, but in terms of total vehicle efficiency --- energy in (or in the case of sunlight, energy available) > vehicle motion --- there are other areas that can yield greater gains.
When we talk about efficiency, changing CD by a few points is great. However, if we could change efficiency of the solar cells from sub 20% (at the time of the Honda sun races) to double that, well, twice as much energy for free, that's efficiency gains. To some extent, internal inefficiencies like charging / discharging batteries and the heat created by electronics (hence the need for other inefficiencies like fans), well, those are big, and they are not shown in "drag" graphs.
The point here is all these aspects are important. Aerodynamics is one very important piece of the puzzle, but there are other pieces that are often overlooked because they are not as "visible". Looking only at the outside limits our perspective. Looking only at the inside limits it as well. Thinking 2D limits us just as much. It's a system, so to really address efficiency, we have to address the whole.
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