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Re: difference in boundary between car and HPVs: I don't think it's the size of the machine so much; it's in how small an aero improvement the two camps decide is worth going after. HPVs have only so many watts for their power source so if they can save a watt, they will. Cars have so many excess watts for motive power that saving a watt there will never show up in the gas logs or on a scangauge.
Perhaps an even bigger factor is the basic shapes involved. Many HPVs are designed with forms that aim for laminar flow, if not all the way back, at least 2/3s of the way back. Plus, HPVs are rarely 4-wheelers which helps aero. Car shapes are so far away from achieving laminar that "true laminar" flow isn't even an issue, cuz it ain't gonna happen. Cars do have thicker boundary layers and those boundary layers thicken as they go back too. As such I don't think surface finish is as critical.
Re: body gaps: I think each gap is a YMMV thing- is it parallel or perpendicular to the wind? Is the leading edge from the "back" of the gap standing proud of the trailing edge or is it flush or is it set in a bit? Is the cavity behind the gap such that flow isn't really happening to any degree... or is there a lot of flow through the gap? I'd surmise if there's flow then that could be a draggy gap.
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