Discuss: can aftermarket window deflectors affect fuel economy?

[DoctorDawg]

OK, I'm usually highly skeptical of any really cheap, easy mods that can yield a noticable fuel economy boost overnight, but here's the story. Last Sunday I installed Weathertech deflectors on my '09 Jetta's front windows and on the moonroof. Then, Monday morning, I set a new all-time record on my 25 mile mostly highway drive in to work...49.1 MPG (reported by MFD, which is proven quite accurate on my car). Previous best was 47.9 on this route, so this was a pretty significant increase. And on the way home that night I set another personal best for that leg of the trip, by about 1.5 MPG. This trend has held up throughout the week so far, such that today the to-work leg of my commute achieved 50.1 MPG. I have never before broken 50 MPG over any significant distance.

I'm not driving any differently, I swear. I'm also not setting my windows any differently than I usually do...in this nice weather I always have, and still do, crack my driver's window about 2", do the same for the rear passenger-side window, and tip the rear of the moonroof open; same now as it was before I added the deflectors. I should also mention that the sudden improvement happened two-thirds of the way through a tank of fuel, so it can't be explained by unusually good fuel, either.

With 2OK+ miles on the clock, and with this apparently consistent improvement happening literally overnight, I ain't buyin' that this has to do with engine break-in. But I'm also not ready to believe that this has anything to do with those deflectors, either....stuff that seems too good to be true usually is. In fact, I'm really not ready to completely believe that I have a fuel economy improvement at all until I have a few tank-fuls of data under these new conditions...stay tuned...but I am curious to know whether any others here have had similar experiences?

Disclaimer: I am not a shill for WeatherTech. It ain't no sweat off'n my nose whether you buy their products or not.

 

[Sip'n Diesel]

has the temperature there increased lately? perhaps you got lucky and had a tail wind in both directions. have you been working out? maybe you just shed some weight

IMHO - the increase you see is likely a combination of factors/variables you can't control, combined with engine break-in, and your consistent style of driving

OTOH - I've got these WeatherTech deflectors too and I can get 60 MPG if I try (correlation is not causation, though!)

 

[40X40]

Good for you, Doggy! I did see an improvement at around that mileage on my car(a manual), but not on my wifes' car(a DSG). Either way, whether it is the weathertech or not, you have a win-win situation!!

Do they help reduce noise?? Are they easy to put on? How do they attach? Is it tape or something else? Does it rub on the window glass or weatherstrip? Did you have to trim them to fit?

Ah heck, just tell us more about it!


Bill

 

[Sip'n Diesel]

IMHO, they greatly reduce noise when the windows are cracked/partially open. if the windows are fully open, then no dice... well maybe some difference but not significant

also IMHO, very easy to put on. they attach by flexing them and inserting directly into the window channel. no adhesive at all on the fronts. they are designed to be a perfect fit. the rears use only a small strip of adhesive on the vertical borders of the deflector. much better design when compared to "VentVisors," for example, which would fall off if it wasn't for the adhesive covering the entire deflector border. even with all that adhesive, in my experience, the VentVisors I had on a GM were crap. they fell off after 2 winters

no rubbing on the glass/weatherstripping and absolutely no trimming/modifying in any way!

 

[lovemybug]

I don't know if mine are brand name or came from VW, but they were on my car when I bought it.

 

[DoctorDawg]

The Weathertech window deflectors are pretty easy to install...you just flex 'em a little and snap 'em into the track; no adhesive, no screws, no nothin'. They fit my car perfectly. Immediately after installation they will give the anti-pinch mechanism on your windows the heebie-jeebies (because yes, they do rub on the glass a mite, which anti-pinch interprets as something blocking the window), so the window won't close fully on its own. You just have to clamp the window between both palms on its way up and give it an extra push, then it closes. After 12 hours closed like that, its not a problem anymore (presumably the gasket material in the track compresses a mite).

The deflector for the moon roof can be another story. Two metal hooks hook into the front corners of your moon roof cavity, then two screws attach the deflector to those hooks. It requires a lot of downward pressure to reach those hooks with those screws, because the deflector needs to be simultaneously bent to conform to the roof's curvature. This is next to impossible to do if you're working alone, with a screwdriver. It is somewhat easier to do if you have a helper pushing down on the deflector while you engage the screw with a screwdriver. But it is really easy to do all by yourself if instead of a screwdriver you use an electric drill with a driver bit, which makes it much easier to apply all the pressure you need all by yourself.

I don't know how Weathertech compares to other brands. I was (and still am) impressed by the quality, fit, finish, and thickness of the products. They reduce wind noise modestly at the windows and a lot at the moon roof. Mostly I installed them to be able to park in the sun with the windows cracked even when afternoon thunderstorms are a possibility (which is like every day in the summer here in North Carolina). No more climbing into an Easy-Bake Oven after work.

 

[Sip'n Diesel]

I don't know if mine are brand name or came from VW, but they were on my car when I bought it.

IIRC VW makes a very high quality OEM deflector. I don't know anything about them, you may very well have some

DoctorDawg - absolutely no adhesive, not even on the rears? if so, then there are likely some significant design differences between MKIV and MV. don't worry about the anti-pinch mechanism too much. over time, it seems my car has "learned" to be gentle? that, or maybe things have just loosened up a bit. the only time I have a problem with anti-pinch is usually the rear windows and not the fronts. seems like the more you use 'em the less problems they give ya... now if I can just find some doors with crank (manual) windows I'll have the perfect setup!

 

[DoctorDawg]

I recall reading in another thread here that the VW deflectors are Weathertechs.

 

[Sip'n Diesel]

I recall reading in another thread here that the VW deflectors are Weathertechs.

hmmm... that does ring a bell now that you mention it

 

[DoctorDawg]

has the temperature there increased lately? perhaps you got lucky and had a tail wind in both directions. have you been working out? maybe you just shed some weight

No, no, no, and no. But maybe I should check to see whether somebody heisted my spare!

 

[DoctorDawg]

DoctorDawg - absolutely no adhesive, not even on the rears? if so, then there are likely some significant design differences between MKIV and MV.

I can't speak for the rears, as I didn't buy them...just the fronts (which, as you know, are adhesive-free).

 

[Fix_Until_Broke]

DD - take them off and see what your MPG's do.

If you can make your MPG's go up/down 2-3 times then you're probably on to something (what I don't know). If there's no correlation between weather you have them on/off and your MPG's then it's obviously something else.

 

[shizzler]

I'm also not setting my windows any differently than I usually do...in this nice weather I always have, and still do, crack my driver's window about 2", do the same for the rear passenger-side window, and tip the rear of the moonroof open; same now as it was before I added the deflectors.

To me this is the key. You always crack your windows about 2"? How far do the deflectors come down on the window? I am geussing about the same distance. Before you had the weathergaurds, this window cracking would probably cause a turbulent disruption of the air flow coming past the vehicle. With the guards on you effectively shield the air flow from this effect. I bet you get a lot less breeze into the car too.

Any object on the exterior of the vehicle that affects air flow is going to have an effect on fuel economy. Normally I would say these gaurds definitely NEGATIVELY affect fuel economy. They stick out past the car into the air stream and add frontal area. But with your window useage, you might have found a reason for them. If you want best FE, ditch them and keep your windows shut!

 

[Sip'n Diesel]

heh heh, now we just need a guinea pig

I'll keep mine on the car, and keep my windows open... I don't use A/C. also, I can't do much better than 60 MPG without turning the engine off

 

[DoctorDawg]

Update: the exceptional fuel economy I witnessed immediately after installing my deflectors (see OP) has since gone away. And since the deflectors haven't (gone away, that is), I'm guessing they had nothing to do with it.

Sure would like to know what changed there for a few days that was giving me 50+ MPG. I could get used to that.