What are the effects of dams, block panels, and side skirts when used together?

[Chopper678]

I have a few new ideas for aerodynamic mods to the truck but I want to keep my mouth shut until I'm more educated so I came here with some questions.

1)What is the purpose of an air dam or side skirts?

According to my research it is to direct air above the truck or around it so that air does not pass through the rough undercarriage. If I am correct, shouldn't a full belly panel eliminate or diminish the effects of air dams or side skirts? How do these work in conjunction with eachother?

I ask because I have seen modders report little to no mileage difference with side skirts. I feel like one reason may be they are straight and not curved to an S shape like semi trucks use.

2) Is it better for air to flow above the truck or below it?
This answer should affect how dams, skirts, and panels are implemented on each individual vehicle. What if the bottom is fully paneled and the top is not? What if I have an aeroshell?

3) How much of my grille can I block before it becomes a danger to my engine or its life? I live near the Gulf where it is hot 10 months out of the year.

Hopefully we can get some progressive discussion going and I can begin my aeromodding journey.
Thank you!

Intro placed at end so you don't have to read it:

Hi all,

I'm crossing over from TacomaWorld.com where I spend a lot of time reading and talking about my truck which I'm very enthusiastic about. I take my truck offroad from time to time but I don't think that should get in the way of being at least a little more aerodynamic.

I started a thread here on EcoModder last year asking about tires, unfortunately it had no responses and hopefully this one is in the correct section.
I have been lurking ecomodder for quite some time now and I enjoy reading everyone's projects but I have noticed a strong trend: Not many people seem to post their MPG gains from various mods (Side skirts, air dams, truck wings, and the sort) after posting up how they made their mod and what it looks like. I know there is tons of it on this site, but the certain threads I lurk that pertain to my truck don't have any sort of testing. I like to test all of my mods with my ultragauge and an excel file.

As for those tires, I went from 265/50R20 Falken street tires w/chrome (80 lbs total weight) to 265/75R16 Bridgestone Dueler Alenza Plus LRR tires with TRD FJ rims (70 lbs total) and gained 2 MPG per tank. The official highway gains (Including Ultragauge video) are kept in record on an excel file that I have not yet taken the time to complete. I still need to watch a lot of recordings and put them in a table and graph which someday I will do.