side skirts under car?
 

Where should I mount the side skirts. Inside the wheels or on the outside edge of the body. I believe bashoos has his inside. Any advantage to having both?

I've seen them mounted on the outside edge of the body most commonly, but why, I do not know the details. I'd mount them on the outside and supplement them with low-hanging rear wheel skirts, to keep the airflow nice and un-interrupted.

The reason is to smooth airflow under the car. I theorize that mounted inboard the wheels would reduce buffeting under the car. I may make 2 sets on for in and out.

I mounted mine inside because it was easier to mount them there and also to keep the jack lift points accessable. Mine are in place mainly to keep the air flow linear under the car between the two axles. The biggest effect I noticed was the reduction in the amount of dust kicked up by the car when driving down a gravel road. Mine are made of coroplast, cut and mounted with its slots running front to back.

That would indicate smooth or low airflow near the gravel, wouldn't it? Which is a good thing.