So, the EV1/Impact started with a Cd of .188. This drops to 0.167 with a mirror delete, short springs, and some duct tape, and 0.153 with a 16" boat tail.
That's a 11% improvement from stock to race aero, then an 8% improvement in Cd by adding the boat tail.
Hmm.... my car has more room for improvement at the rear than an EV1 ever had. I bet about 20% of my 0.25 Cd results from the trailing wake.
I'm just going to look at this picture and ponder for a while. Maybe I'll sit down with some cardboard and that spare rear bumper cover I bought.
How badly do I want 70mpg @ 70mph?