I could be wrong on this, but wanted to see if others are seeing what I am. I have the '08 MMH 4wd with the sunroof. The roof rack crossbars are removed. I have noticed that if I crack the sunroof open (moving the glass back to leave approximately a 2-inch gap, not tilting the glass) the mileage has been increasing around 1.5 MPG. Then I decided to try it for a tank. With the roof closed I got 32 MPG. With it open and driving over similar roads, 33.5 MPG. Outside temperatures are very mild now, so it's not related to the extra ventilation affecting heat or a/c operation.

This seems counterintuitive. I'd expect this to create drag. But I've run a test on this a few times on a familiar stretch of road, and the gain seems to be real. This is a significant gain. I'd like to do without the extra road noise this causes, but for 1.5 MPG it's hard to resist. I find it unlikely Ford could be leaving MPG on the table in their wind tunnel testing, so I have some skepticism. If true, I'd gladly buy some kind of wind deflector that accomplishes the same effect.

If others try this, please post back with your results.

 

How are you calculating your mileage? By hand? By scan guage? By Ford Navigational system detail? Other? Are you driving the same road at the same time of day? With a warm or cold engine? I don't know the reason why just the open moonroof would enhance mileage.

 

There is one possibility that I can think of. If your fan is on, you are potentially pushing air out the top of the vehicle. This actually creates some lift, thus reducing rolling resistance. I'm skeptical that you would see that much of an improvement, but the theory is sound.

Incidentally, this is being investigated for trucks, and is said to reduce the load on the wheels by up to 15%.

If you wanted to test it, find a hill, coming down it, get up to speed (say 65), and then coast. Every 10 seconds capture your current speed. Repeat with your moonroof open. If the profile is different and you are faster with the moonroof open then your drag is reduced.

 

I tend to drive with the fan on a moderate setting, not full blast. Also, the way I calculated the number was by using the nav system display, resetting it when opening or closing the sunroof and running the vehicle on flat road where I can get ten or 15 minutes of steady readings at around 63 mph.

If the gain is due to the fan, running it at full blast should produce a greater boost so I'll give that a try next time I'm on the road.

Admittedly, I'm not doing a scientific test. But with the many experts here who might have more equipment and better places to test, I thought it worth sharing (and worth risking the embarrassment of being proven wrong.)

 

I've thought for years about why cars don't have a lightweight airfoil on the roof to 'lift' the vehicle, thus reducing weight. Yes the airfoil would add weight, but w/lightweight composties available these days, one would think the lift created could counter balance itselft, plus provide some more lift. I may be totaly wrong tho. But I'm sure from a safety standpoint I'm sure no manufacturer would support it as well due to less weight = less traction = higher potential for accidents.
I'm also not sure that just having a 'wing' a few inched off the roof would allow for lift to happen since the restriced space between the roof & the bottom of the 'wing" might also increase airflow, thus reducing the lifting properties of the wing.

 

While that would reduce downforce and perhaps rolling resistance... wouldn't it increase drag?

 

Turbulent air has less drag than calm, steady air.
The skin on a shark is rough, and this cuts through water easier than something smooth, like a boat hull. Race boats now are making rough skins, but on a sub millimeter level.

Also, look at golf *****. ***** with dimples travel farther than smooth ones.

The moonroof may make turbulence to some degree.
I can't imagine it equalling 1.5 MPG boost, but you never know!
By pure chance, you may get a winning combination of air turbulence!

 

I've thought about posting on the difference I get w/my moon rook open too. Mine is not as significant as yours. It only seems to be about .5 mi increase traveling the same road/route every day. But the mpg is better. My best tank ever was a week I was able to keep the moonroof open the majority of the time.

And to think the first month I was afraid to open the moonroof because I was trying so hard to up my mileage. Now I look forward to the days it will cool enough on the drive home to use it vs AC.

PS - I have the FEH 4wd, but haven't removed the bars from mine.

 

Anyone tried testing the effects on MPG when you have bumper stickers on vs off?
You could apply stickers on the side windows too for more turbulence.

Duncan