I agree--it is more decorative and hopefully functions to keep the rear window a bit cleaner.
It is such lightweight plastic, I doubt it affects mpg. I also wonder if it serves any aerodynamic function due to its location.

 

The perception here is that a Spoiler is there to create downforce when this may not be the case. Many Hybrids have spoilers (although not as large as the fit's) Downforce is not an issue at speeds that the fit realistically can be driven at so it would have to likely be there for other reasons than "downforce", especially since it doesn't "point" down. It (like on many SUV's & Vans) is likely there to reduce drag by disturbing the vacuum that is created behind the vehicle. My Integra's Rear spoiler did this for performance & Economy reasons.

The economy is likely more due to Lighter wheels that have less rolling resistance. Lowering the weight of the rolling stock will do wonders for economy (as well as performance) and narrower tires will cut down on the resistance so while the weight has a little bit to do with the economy, it's probably not the body kit but the tires/wheels that makes the difference between the two. (although lighter weight anywhere never hurt - or so my wife says)

Agreed The more you can cut down on Resistance, the better economy & Performance you will achieve. Like putting lighter wheels on a car, you are not actually increasing power, but removing/lowering what is limiting the power/economy the car is capable of.

Well... I agree with everything except for the Tailgate theory... Air inside the bed actually just swirls around and doesn't really leave the bed of the truck so it has little impact on the economy. anyone with a truck that has had leaves in the bed has seen the same leaves swirl around in circles, one over the top of another, and not get sucked out of the bed. (naturally some do get sucked out but it's not like they all go out at once. They can swirl around for tens or even hundreds of miles depending on speed.)

~SB

 

Doubt it creates any downforce like a true automotive wing, but does it change the air flow separation length?

 

I think it might help with the efficiency. I remember reading that those wings installed at the end of a wagon will help reduce the abrupt end of the airflow and cause the car to drag less vacuum behind it. The fit is basically a wagon, so it would apply here too.

The added weight would definitely hurt. No down-force is possible because of the angle. It does look good and does serve as a sunshade. Also in the rain it keeps the upper part of the glass cleaner.

 

ok I remember swapping my spoiler out for my JDP carbon and it couldn't have been more than 5lbs tops, so I don't think weight is a factor at all.

 

with proper application... science suggests it'll help and not hurt

 

Where Mythbusters failed was the way they measured the fuel being used. They used a flow meter between the pump and the pressure regulator. they correct way would have been to find a truck with a carb or to use two flow meters; One on the high pressure side and another on the return line downstream of the pressure regulator.

As for the "spoiler", Looks only and I think if I took mine off I might see a cleaner rear window when driving on wet or dusty roads.

 

I suspect it may help gas mileage but only in the slightest, unnoticeable amount. It helps break up the vacuum (drag) behind the car. I suspect its main function is to help keep the rear glass from loading up with dust, dirt and rain.

 

Let's take out ideal aerodynamic shape and think about how the spoiler makes the fit closer to said shape:



A part of the reason why small cars like the fit get the same gas mileage on the highway as larger cars like the civic is due to the short length of the car. Because the fit ends so abruptly, it doesn't have time to complete the ideal aerodynamic shape, and drag suffers. Drag is not a factor at slower speeds which is why we see small cars outperform in city mileage but not highway. Same reason the smart car gets such shit highway mileage.

Porsche finally figured it out with the new convertible:

 

That Porsche reference reminds me of the episode of Mythbusters.

YouTube - Mythbusters - Reverse Engineering - Part 1
YouTube - Mythbusters - Reverse Engineering - Part 2
YouTube - Mythbusters - Reverse Engineering - Part 3