Think twice before putting one on your Civic.
Spoilers are very prominent on a lot of cars. It's a fascinating phenomenon, as spoilers aren't really as useful as you might think. Do you even know where the term "spoiler" came from, kid with a five-foot piece of plastic on the back of his stock Civic? To start from the beginning, as you're traveling at 90 mph down the highway (purely hypothetical of course) your car is experiencing drag, which is air pushing the car down. Spoiler or not, every single car under the sun experiences this.
Because of the way the air pushes the car down, there are very specific things you can do to disrupt, or, "spoil" the airflow around the car. One of which is a spoiler. A spoiler is a flat flap designed to decrease drag so your car will travel faster without having to really add anymore throttle. As such, it makes your car more fuel-efficient. The natural order is for air to engulf your car as it's going forward. A spoiler redirects air that hits it, that is to say a spoiler says "don't come here" to the air and so it goes elsewhere, therefore less air is pushing the car down, and thereby more speed is obtained. On the opposite end of the spectrum is a wing, which increases drag. It's a flap that is more vertical, and therefore captures air to keep the car grounded.
You see these a lot on F1 cars, because at 200 mph in a car that weighs just over 1,000 pounds you need drag, and a wing will make sure the car doesn't flip over. A wing and a spoiler differ in design. A spoiler will have flat flaps, where a wing will have flaps that are slanted. F1 cars use something called DRS, which stands for "drag reduction system". The driver presses a button, and the wing on the back changes from vertical to horizontal, going from a wing to a spoiler, because if he wants the car to turn well at higher speeds he wants more drag, but when he wants to pass someone on the straightaway he wants less drag. And now you know why that is, because of the nature of a spoiler and wing's shape.
While this is an essential thing to have on a race car, for a car that goes no higher than 120 mph and takes minutes to reach that, the car just isn't fast enough to utilize the wing. But who in their right mind would want more drag on an already slow car anyway? We know the downside of having a wing on a street car, but what's the downside of the spoiler? All it does is decrease drag, so the car should go faster right? In theory, that's correct. But if all you have is a spoiler on the back of the car, then you'd be creating something called lift. Unless you're in an airplane, you don't want lift. See when you add a spoiler, the balance of airflow around the car is disrupted, so the air underneath the car becomes much more prominent, and will cause your car to lift up.
So in order to make your spoiler effective, that is to get the balance back, you'd need something else. A front spoiler goes on the front of the car, and is found below the bumper, which means it will disrupt airflow to the bottom of the car. It's pushing the lower air back to the top of the car, and therefore decreasing lift and increasing drag again. Again though, this only becomes a problem for really really fast cars. Armed with this knowledge you can spot people on the road who only have their wings and spoilers for show purposes. In short, get a spoiler only if you have a fast car and if you're going to drive it really really fast. And this isn't multiple choice. You must pick both.