[azraelswrd]

I'd focus on the tires before aero then because you're not driving very fast. If you already did that (and forgot to mention it) then I would start with the easiest aero mods and that would be to clean up the front or initial point of fluid contact. Anything "unsmooth" will need to be smoothed out or over.

I think your plan of aero mods are sound -- clean up the underarea then address the rear-wheel area. Search the forum for how others have done it but before you go nuts and start spending $$$ I suggest testing and a monitoring device/system because driving habits can ALWAYS be improved upon and this is the best/easiest way to jump the MPG hurdle.