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roof cargo box vs. MPG
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targis58
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PostPosted: Sun Feb 08, 2009 1:19 pm    Post subject: roof cargo box vs. MPG Reply with quote

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So.. I have been carrying the box for a few month and I was wondering how much it affects the MPG on the van...
Has anyone actully calculated/tested the MPG difference?
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PDXWesty
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PostPosted: Sun Feb 08, 2009 1:25 pm    Post subject: Reply with quote

I have not calculated the mileage difference myself, but I only get 15-16 mpg both around town and on the highway. When others report 18-21 mpg, I don't know how they do it. I have a very good running tuned 2.1, but my cargo box is always full and I carry a lot of weight on board all the time. I have a feeling all this extra weight and the aerodynamics of the box and rack contribute to the lower mileage.

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mmdingman
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PostPosted: Sun Feb 08, 2009 1:31 pm    Post subject: aero Reply with quote

my experience is that the vanagon is such an aerodynamic brick to begin with, I cannot detect a difference w/ and without (could also have something to do with the accuracy of my fuel gauge). I have some objective equations concerning MPG w/ and w/o a ski box in an aerodynamics book... will dig up.
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Crughy
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PostPosted: Sun Feb 08, 2009 3:01 pm    Post subject: Reply with quote

Like all drags, it gets exponentially worse with speed.

Might not make such a diff a 60 but at 70 and over, it might hack more than a couple of mpg.

JP
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levi
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PostPosted: Sun Feb 08, 2009 3:28 pm    Post subject: Reply with quote

Here's two vans for you to compare:

It's informative and not at the same time, as you'll soon discover.

The engine is identical, since I installed it in one, then removed the engine and installed it in another van.

First was an 86 pop-top with overhead rack almost identical to targis58.
However this had an auto tranny.

Pulled that engine and put it in an 85 westy/AWagen. 4 speed.

The 86 never returned better than 17-18 city, 14-16 hiway.
The 85 never gets worse than 21-22 city, 21-25 hiway.

All numbers from GPS.

Notable difference is tires.

The 86 auto had wide, soft compound Nokians, and I suspected they may have been out of alignment, since the outside 1-1/2" of the fronts wore considerably faster than the rest of the tire. However I did take it to a shop for alignment and they said it was okay.

The 85 has skinny hard compound tires.
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jackbombay
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PostPosted: Sun Feb 08, 2009 3:40 pm    Post subject: Reply with quote

I'm sure some of you feel the same way as I do about ODOs, that is unless the ODO is know to be accurate, I assume it is inaccurate.

Of the 6 cars I have owned in my life only one had an ODO that was within %1, that was a 1978 chevy motorhome.

I typically only trust GPS speed and mileage numbers, unless the ODO is regularly compared to 50+ miles of highway mile markers which I do often, I have plenty of time while road tripping so why not?

As far as the box/MPGs go, the front bin on westy's RUINs the aero so the box probably won't affect MPGs much, IMO.
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WestyBob
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PostPosted: Sun Feb 08, 2009 5:21 pm    Post subject: Re: roof cargo box vs. MPG Reply with quote

[quote="targis58] I have been carrying the box for a few month and I was wondering how much it affects the MPG on the van...
[/quote]

I think the culprets have been identified: speed, tires, engine health, brick body.

Around town the carrier affect should be negligable since the engine isn't straining to move air out of the way for it.
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targis58
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PostPosted: Sun Feb 08, 2009 6:50 pm    Post subject: Reply with quote

jackbombay wrote:


As far as the box/MPGs go, the front bin on westy's RUINs the aero so the box probably won't affect MPGs much, IMO.


Anyone found the solution for this? any wind deflector for the front bin?
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MrPolak
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PostPosted: Sun Feb 08, 2009 7:23 pm    Post subject: Reply with quote

Speaking of aerodynamics:

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tds3pete
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PostPosted: Sun Feb 08, 2009 8:09 pm    Post subject: Reply with quote

Years ago we had a small corporate fleet of VW Dasher diesel wagons. They were all identical, except some had the factory roof racks. The ones with the roof racks got around 2 mpg less on the highway.

We were surprised to see that much difference for just the bars running front to back.

Not a scientific test by any means, but a measurable difference.
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dixoncj
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PostPosted: Sun Feb 08, 2009 8:40 pm    Post subject: Reply with quote

We have an '02 1.8t Passat Wagon. With regular surf racks on it, it gets about 5mpg less at highway speed. W/our big aerodynamic Yakima box, it gets about 8-9 miles less. Big damn difference. You can really tell what a diff aero's make on a car that's the shape of a dart vs a brick. I take both off when we go on a long trip unless they're really necessary.
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?Waldo?
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PostPosted: Sun Feb 08, 2009 8:46 pm    Post subject: Reply with quote

I've been curious how much of a difference mud flaps make.

Andrew
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madspaniard
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PostPosted: Sun Feb 08, 2009 8:50 pm    Post subject: Reply with quote

Andrew A. Libby wrote:
I've been curious how much of a difference mud flaps make.

Andrew


south african or german? Laughing
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jackbombay
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PostPosted: Sun Feb 08, 2009 8:55 pm    Post subject: Reply with quote

tds3pete wrote:
Years ago we had a small corporate fleet of VW Dasher diesel wagons. They were all identical, except some had the factory roof racks. The ones with the roof racks got around 2 mpg less on the highway.

We were surprised to see that much difference for just the bars running front to back.

Not a scientific test by any means, but a measurable difference.


TDI Jetta sedans have no roof racks and were EPA rated at 49 MPG, the Jetta TDI wagons have factory roof racks and are rated at 50 MPG, fwiw.
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dixoncj
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PostPosted: Sun Feb 08, 2009 9:02 pm    Post subject: Reply with quote

Jack - the roof racks on the jetta wagon just like the passat - runs front to back only out of the factory. You put a pair of crossbars on that sucker - with surfboard pads or other stuff - and mileage is going to drop.
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jackbombay
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PostPosted: Sun Feb 08, 2009 9:08 pm    Post subject: Reply with quote

targis58 wrote:
jackbombay wrote:


As far as the box/MPGs go, the front bin on westy's RUINs the aero so the box probably won't affect MPGs much, IMO.


Anyone found the solution for this? any wind deflector for the front bin?


Cut it out, build a masonite "form", and fiberglass it over. Thats what I did, less wind noise on the highway too. I changed wheel/tire size at the same time so its hard to say how much of a difference the bin elimination alone made, but I picked up 2-3 MPGs on the highway.

Last trip loaded to 5200 pounds I got 34 MPG at 67MPH (speed and distance GPS verified) and it is really hard for me to ever drop below 30 MPG on the highway under normal conditions.

I get 31 MPG in city driving never getting above 35 MPH, I do have an E-TDI. I've never read any 1st hand MPG numbers as good as mine, fwiw. My best tank was 37 MPG. This spring I'm going for a 40 MPG tank, at 55 it should happen.

That \/ is what I was referring to when I said the bin ruins aerodynamics.

MrPolak wrote:
Speaking of aerodynamics:

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jackbombay
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PostPosted: Sun Feb 08, 2009 9:12 pm    Post subject: Reply with quote

dixoncj wrote:
Jack - the roof racks on the jetta wagon just like the passat - runs front to back only out of the factory. You put a pair of crossbars on that sucker - with surfboard pads or other stuff - and mileage is going to drop.


But the dasher racks are the same as the jetta racks IIRC, front to back, right?
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targis58
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PostPosted: Mon Feb 09, 2009 7:07 pm    Post subject: Reply with quote

dixoncj wrote:
We have an '02 1.8t Passat Wagon. With regular surf racks on it, it gets about 5mpg less at highway speed. W/our big aerodynamic Yakima box, it gets about 8-9 miles less. Big damn difference. You can really tell what a diff aero's make on a car that's the shape of a dart vs a brick. I take both off when we go on a long trip unless they're really necessary.


Wow it's amazing how just racks without box can cause 5mpg difference!! Shocked

Was that WITH or WITHOUT a wind fairing?
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dixoncj
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PostPosted: Mon Feb 09, 2009 7:10 pm    Post subject: Reply with quote

Without - it's brutal. Blow's my mind. Just shows how big a diff aero's make at 70+mph.
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reluctantartist
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PostPosted: Mon Feb 09, 2009 8:04 pm    Post subject: Reply with quote

Jack when you say "Cut it out, build a masonite "form", and fiberglass it over." Did you just build a cover to cover the indentation or did you remove the rack all together?
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