The majority of spoilers on the market are just for looks. Race cars are fast and they have spoilers, so people think that any car with a spoiler must be fast. There are some street cars (usually modified) that could benefit from additional downforce of a real, functional spoiler, and there are aftermarket suppliers who will be more than happy to sell you one. Also consider this; most modern cars are aerodynamically sound enough on their own to not really need a spoiler at any legal speed while many older cars (like my tail-happy '89 Golf) are boxy and non-aerodynamic enough that a spoiler might help. Response to heyitsmatt - On most street cars, the added weight of all that plastic/fiberglass will likely provide more downforce than the aerodynamics of their non-optimal shape.However, when you consider that many of the cars with these "fashion" wings have unneeded 5-inch exhausts (which may REDUCE engine power), actual performance apparently is not a priority, rendering the point moot.

A properly designed spoiler can improve the aerodynamics of a vehicle and improve its fuel consumption. Because car manufacturers have to comply with strict fuel consumption guidelines, I expect that typical factory equipped spoilers are at least neutral, and may help fuel consumption at some speeds. At freeway speeds, traction improvement from the downward force effects will be relatively insignificant. Most of us never do high-g cornering at freeway speeds anyway, and a rear spoiler won't help your braking since in emergency braking more than 90% of your stopping depends on your front brakes. Bottom line: --almost-- exclusively for looks.

Most of time they are for looks. A decorative addition. Spoilers are a cosmetic enhancement that do nothing for most cars. A high percentage of today's cars are front wheel drive. Spoilers were originally meant for high performance cars. For street cars, save your money!!

Most of this is in response to Bob Blaylock, who has a misguiding answer. Your stated function is not correct. The function is not to give the drive wheels more traction! Spoilers provide aerodynamic stability. For example, by having some downforce on the rear end, you increase braking stability, as the car wouldn't tip forward as much, helping to keep the rear end planted. You extremely exaggerate "simple, obvious facts". The facts that you present are anything but obvious. 1) The front-wheel drive argument is wrong. You don't want the downforce at the front wheels. At highway speeds, most street cars can't exceed tire traction limits by accelerating or slowing down. So it's pointless to add downforce over the drive wheels to aid traction. Instead, downforce stabilizes the car: it reduces the wavering of the suspension and helps keep the car planted. Furthermore, if you did add more downforce to the front, you would further unload the rear end of the car. Given that most front-wheel drive street cars are heavily weight-biased to the front, shifting that weight further forward will make the rear end more likely to loose traction and cause a spin. 2) I have sat on car spoilers, and while I am not proud of the fact, they have sustained collective weights of our friends in the high 300 lbs range just fine. Also consider that the downforce builds gradually. The trunk lid is actually pretty strong, and mount points for spoilers distribute weight appropriately. So it's OK and the trunk lids don't crumble: that's why you see touring cars (e.g. SCCA World Challenge) running with monstrous-looking wings that are screwed on to an otherwise stock trunk. 3) You fail to quantify what "significant" is for the downforce. 5% can be significant if it means that the balance of the car is shifted that much to the back. I don't have numbers on what is significant. Now, the real argument is that most OEM spoilers are for looks and not performance. This is because of their shape: they are not angled appropriately and not positioned to create a surface for downforce. There's a reason why racing spoilers sit higher on the trunk and have adjustables angles. So while you may get some improvements on the highway from a large spoiler (usually not OEM) it won't make a difference around town, and so you should save yourself the trouble.

So my question is: Doing 65-70 in my FWD Kia Rio, does the spoiler do anything? More stability? Waste gas? Etc.?

They make very useful picnic tables.

I'm just getting into this overall subject in an attempt to improve fuel mileage on a minivan. So far, my reading indicates that there are two devices that are commonly called "spoilers". The first type is a "wing" which is designed to place downward pressure on the rear of a vehicle to increase traction. That appears to be what most people are talking about in this thread. The other is a true "spoiler" which is designed to "spoil" the turbulance at the rear of the vehicle by straightening out the airflow over the rear area of the vehicle, and that can give a slight boost to fuel mileage. The descriptions I've read mention that by straightening the air flow, the rear window of mini vans stay cleaner as a side benefit. That rings a bell because I've read the same thing in minivan brochures at new car dealers. DWeb99

The spoiler will waste a little on fuel consumption but not a great deal as its causing drag and you have just added weight to the car but remember on front wheel drive cars if you start doing silly speeds when cornering trust me you will lose the front end as the spoiler is pushing down on the back of the car and lifting the front so your steering becomes alot lighter which will cause oversteer.

Most of you are all wrong. The spoiler on a car is different from a wing. WINGS are the big gigantic "spoilers" in the rear of high performance race cars. THOSE are designed to provide downforce. SPOILERS are NOT designed to provide downforce AT ALL. Spoilers on cars are much smaller, and typically look like a small but sharp upward slants at the rear. These are designed to trip the laminar flow boundary into turbulence over the rear of the car, so that the flow does not separate as much. Its behavior is similar to that of the dimples on a golf ball, and actually reduces the drag on a car. A good example are the turbulators on top of the lancer evo 9. If you don't believe me, wiki "boundary layer."