Wow... 0.25 Cd. I wonder how much of that is because of the FULL SMOOTH-SURFACED UNDERBODY PANELING?
Looks like they don't consider the hubcaps a priority, opting for weight over aero.
I never gave that gap much thought but now that you mention it and seeing that pic... you're making me a believer. I recently removed my ad hoc smooth hubcaps and rear skirts because they weren't having any effect on my routes but this sounds easy/small enough for me to squeeze in before I get back to my real job of schooling.
Does Prius have a "show me" diagram like that? I'd be interested in seeing all their upgrades, plus those from the Volt. Time to call upon the mighty search engine gods!