I understand a little about the uses of spoilers. However these question refer to different topics of adding spoilers to cars that don't come with them. I'm also aware that some people put on spoilers for looks and style.
1) Would putting a rear trunk spoiler or tail wing on a Ford Crown Victoria (Police Interceptor) make it more stable, safer, faster, better gas mileage, break wind?
2) Same question for a full size van like Chevy or Ford?
3) My 2005 Toyota Sienna has an option to put a spoiler on the rear hatch back of the van. What would that do to my van or is it just style?
4) Cars that install low spoilers such as skirts, Etc., besides style, does it do anything to the car?
5) Sports cars that already come with spoilers top and bottom...but that are changed for different and possibly bigger ones. Is it mostly style or do they work in stabilizing, fuel economy. and breaking wind?
Basically for those type of vehicles spoilers are kind of just for show. Race cars go over 200 mph it does stuff for them. Gigantic diesel trucks are very on aerodynamic and putting spoilers make them get a little better fuel because they're so huge and push so much wind. Her nails vehicles you have there more for show if you decide to pull one on
Generally you don't want add any of that unless it came from the factory like that.
I knew a guy who put both on his goofball "transformer" neon & the wing actually lifted the rear end off of the ground at highway speeds because he just slapped it on not realizing these spoilers/wings need to be tuned for the specific vehicle.
That being said, none of those vehicles need spoilers. This resorts back to what comes from the factory but you'll see some minivans with tiny spoilers at the rear of the roof.
This is because the original manufacturer found in needed rear downforce as it was unstable. They're always angled on those to force the wind down onto the rear bumper.
IMO the only practical benefit one can have from spoilers in everyday driving, is that those located above the rear view window can in some cases render significant protection against mud and dust. It is not theory: it is a fact: after I installed one above the rear window of my Audi Wagon, this window stays clean so much longer than before.
Are you talking about the type that mounts at about a 45° angle?
This is a bad pic (Taurus wagon on a Subaru parts site was the closest to pop up) but it's similar to the van type I was talking about. While directing air into the gap where the red line is for added downforce the wind blows light mung away although it's more of a happy accident.
Yes, smth similar to what is on your picture.