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Old 11-06-2008, 11:17 PM   #11 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Whoops! That was a bald faced assumption on my part. (That'll learn me.)

I'm skeptical about the identical Cd though (and/or inclined to believe trebuchet's idea). VW USA shows Cd of .31 for the sedan, but "TBD" for the wagon.
I agree that the sedan and wagon don't have absolutely identical drag coefficients, but I think its entirely possible for them to be within .01 decimal point. In fact I think there is a good chance the wagon is more aerodynamic than the sedan when you drag it out to 3 decimal points. I see no reason to automatically assume that the sedan will automatically be more aerodynamic than the wagon. The back window and trunk lid of a sedan are a very complicated aerodynamic problem that has to be compromised with functionality and visibility. On a wagon the roof makes one continuous smooth curve all the way to the bumper.

I still don't get why the new Jetta wagon is less aerodynamic than a 10 year old Passat wagon. Did VW tear down its wind tunnel recently?


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Old 11-07-2008, 11:03 AM   #12 (permalink)
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Originally Posted by tasdrouille View Post
I'm skeptical about the Cds too. I'm sorry but that's highly unlikely, unless the flow never reattaches to the trunk on the sedan.

Treb, I would really be grateful if you could provide a souce for those numbers!
No source, pure speculation for an example of the large range contained within round off error.


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On a wagon the roof makes one continuous smooth curve all the way to the bumper.
Until it makes an acute angle change to make the back portion of the car o.0

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I still don't get why the new Jetta wagon is less aerodynamic than a 10 year old Passat wagon. Did VW tear down its wind tunnel recently?
I hear you.... I think it comes down to design goals... Or rather, they started off with an in style shape, then put lipstick on it to make it slightly better. When you start from a concept that was designed by people that aren't aerodynamicists...
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Old 11-07-2008, 11:35 AM   #13 (permalink)
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Remember, these numbers are ESTIMATES, not anything carved in stone. They are there to attempt to compare vehicles. As the saying goes: Close enough for government work.
Quirks in the system show up other places. The Saturn Vue Hybrid and Aura Hybrid have identical drivetrains. The EPA rates them both at 32 hwy, but the Vue (a small SUV) rates higher than the Aura in the city (25 vs 24).
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Old 11-07-2008, 01:00 PM   #14 (permalink)
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The data file that RH77 posted is interesting, does anyone know what the 'GUZLR' field is actually saying? It looks like it's only on high end cars, is it really just saying that they're gas guzzlers?
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Old 11-07-2008, 01:24 PM   #15 (permalink)
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Originally Posted by tjts1 View Post
I owned a Chevy Celebrity wagon. It is not 300-400lb heavier than a sedan. The A body sedans and wagons had near identical weight.
I was just going by my experience. Ive actually owned 3 a-wagons and 2 a-sedans. All had the HD brake option, V6, 440T4 automatic w/3.33 final drive, AC, and most other options. All had the square rear window. For kicks once I weighed a sedan and wagon at the co-op, both with full tank of gas, no interior items, and no passengers. The sedan was 3100 lbs. The wagon was 3500 lbs, which included my trailer hitch that was about 40 lbs. Also from my experience driving the same roads for many years, the sedans get about 2 mpg better overall than the wagons. Also Ive found the 3.1 gets about 2 mpg better than the 2.8 with all else equal.
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Old 11-07-2008, 04:29 PM   #16 (permalink)
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I don't really have an answer, but I offer up a little "observational commentary";

I have this type of discussion with my girlfriend a couple times per month (she works in marketing and advertising, I work in science and technology). She's very smart and understands, but couldn't care less about Cd numbers. Hard for many to imagine, yet there it is.

My conclusion from my experiences and conversations is this; the general public does not posess what I call "the engineering mind". To an engineer, the example above seems like a marketing 'freebe'. "this one is slightly better than that one for the following reason". Engineering low hanging fruit, take credit where credit is due for god's sakes. To sales and marketing folks (the team responsible for turning harware into profit) this level of detail doesn't help sell cars to the 95 percentile target market, so they don't bother. Making things worse, the general public doesn't want to see engineering data that they might not understand. She calls it 'picking the fly $hit out of the pepper' that may be, but techno-geeks like myself, most others on this forum, want to undestand the nitty-gritty details, it's how we make sense of the world. As it turns out however, <a shocker> the general public doesn't see things that same way. that's my 2 cents
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Old 11-07-2008, 05:39 PM   #17 (permalink)
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Originally Posted by metromizer View Post
As it turns out however, <a shocker> the general public doesn't see things that same way. that's my 2 cents
I'm totally with you on that.... From my limited experience in product development - I can only agree.

But, that said, if you're going to publish data - it sure as hell be good data.

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