The purpose of a spoiler on the rear of a car is to create downforce (on the driving wheels) thereby increasing the grip of the drive wheels to the road more for better grip in turns than anywhere else. While generating downforce, spoilers CREATE drag, not reduce it. At the speeds we drive in our FFHs, I can guarantee you they are generating way more drag than downforce. Read this: http://www.quora.com...e-back-of-a-car.
There may be some downforce created at 140 miles per hour that Waldo referred to, but MY FFH will never see anywhere close to that speed. Spoilers generate downforce in proportion to speed and it takes a lot of speed to generate that force. Read this also: http://www.physlink....perts/ae496.cfm
And finally, "Most car owners add spoilers to their cars to create a sporty look associated with race cars. Many types of spoilers add a sleek and stylish form to the car, but people who drive in the city or in the suburbs under 70 miles per hour may not see any immediate changes. However, people who do serious highway driving can benefit most from adding spoilers." This statement from here: http://www.ebay.com/...77633228/g.html
Have a nice day y'all!
You're thinking from the race car perspective, not the auto OEM perspective. There are two types of race car spoilers, the ones that are upside down wings, and the ones that are vertical or close to vertical walls that generate a high pressure area in front of them. The OEM spoilers like the one on the Fusion is neither of those. Spoilers on sedans reduce drag because they "clean up" the air flow as it falls off the back of the trunk. Essentially they kick up the air so that it flows over the turbulent zone behind the car, which reduces drag a little bit. It may seem counter-intuitive, but most often a sharp edge at the rear of a vehicle will reduce drag. Look at the back of a new Suburban, they have very sharp edges on the D-pillar for the same reason.
If you don't believe they are effective at normal speeds, then you also wouldn't believe that the underbody shields or the tire spoilers or the grill shutters would make any difference either. All the pieces have small individual contributions, but they all add up.