Just replaced the engine splash shield, which will smooth out the flow underneath. Gets better mileage in winter than my Jeep got in the summer, so I can't complain.
Only had it a month. Stay tuned.
Going to test out vortelators on the A-Pillars and VG's above the rear window, ahead of sun roof, and at trailing edge of the hood. I know mileage increase will be essentially non-existant, but any reduction in road noise and stagnant rain water on the windows will make me happy.
Going to install standalone oil and trans fluid coolers behind the bumper cover, complete with ducting, thermostat controlled fans and linear actuator controlled inlet doors.
Some amount of underbody paneling to smooth out flow, and possibly a rear diffuser to smooth out the flow of the back.