I'll add a couple of things. I think an air dam under the bumper is somewhat problematic. (And I thought that was what you were talking about, although I didn't understand you were talking about fender skirts - shades of the 50s)!
An air dam is going to increase your frontal area. This increases the "flat plate" air drag on your vehicle. But, it reduces the airflow under the vehicle which is quite turbulent. It would possibly be a "wash", but my gut feeling is that it would increase overall drag.
Fender skirts would help a little bit, I suspect, but I would not be able to stand the "look".
I would NOT go to a reduced gear ratio. Yes, the engine will cruise at a reduced RPM, but the engine load is increased. You reduce the effective force (against the ground) that the engine can exert to get you up to speed and keep you at a cruising speed by doing this. The only way to get that "required" force back is by giving the engine MORE fuel at the lower RPMs.
Your Wife may "coast" for a longer period when coming to a stoplight or other stopping situation. Any way you can reduce the use of brakes is going to help your mileage, while possibly making people behind you angry.
Interestingly, Janet uses her brakes more than me, and it drives me crazy to ride with her.
Lowering/leveling would probably help. Again, it has the effect of reducing the overall frontal area effect.
I assume you are using a "high-octane" tune if you're using premium fuel? If not, you are wasting gas. You MIGHT try increasing the timing ever so slightly if you have that capability in your programmer. My understanding is the CS/CTS platforms don't give you that option (and I've never looked for it on mine). But, if you can, try +0.25 degree increments and TEST!
- Jack