The iMiEV may look smooth, but with a Cd of 0.35, it ain't exactly streamlined. That round rear is not optimized for aerodynamic drag, and the slotted wheels aren't necessary when there's such little braking heat to dissipate, due to strong regenerative braking. So, the DIY community has long cooked up little modifications like salt flat discs, fender skirts, air dams, belly pans, duckbills, rear visors, and even boat tails! Here's a rendering of what some of those mods might look like on an i. Here's also a link to a Dutchman who made the best DIY removeable boattail that I've seen.
In grey is the outline of a possible Kammback boat tail. For you sailors, you'll recognize the concept of 'waterlining'. Stretch out the same or even a bit more mass into a longer, more streamlined shape, and you'll reduce drag. This one follows the rear body curves as closely as practical without actually touching, and has a sharp rear cutoff as dictated by good old Mr. Kamm
Here's a graphic of one of the few spoilers scientifically designed for drag reduction rather than downforce. Too bad they're not commercially produced. Time to start whittling!
And the Texas Mile winner in October 2011 chose a simple duckbill for drag reduction rather than a downforce spoiler. If a high po Corvette doesn't lift at 245 mph, d'y'all rice boys really think that a wing's gonna help your Civic?
So, the easiest among the above winners would be to simply add discs to the rims. I went a notch up from JC Whitney's offering, but not quit Mooneyes pricing, and got the Taiwanese-made CCi brand, and am very pleased with the heavy gauge metal (yeah, adds a bit of rotating mass but doesn't dent when I push 'em on), excellent finishing, and positive lock on the wheel.. Still don't know how difficult they will be to remove, but I've got the right nylon pry tools.