A full, complete belly pan would be the absolute best for mpgs. It would be even better if the rear departure angle is around 5 degrees.
You're in luck! I did aerodynamic coastdown testing with my Escort ZX2 a couple years back. I gotchu
Here's the link -
BabyDiesel's Cd testing thread
I noticed that my pictures have broken links for some reason. I'll have that fixed in the morning for you. The charts and diagrams really help in figuring out what's going on.
Also, you will need higher speeds to accurately test the rear pan. I did my testing starting at 55 mph, which was actually low. Basjoos started his testing at 80 mph IIRC (same methodology I used too!)