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Old 11-06-2008, 04:55 PM   #1 (permalink)
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Is VW lying to the EPA? Why does Jetta sedan / wagon have identical MPG ratings?

I was under the impression that the EPA took aerodynamic parameters into account in the final calculations for their dyno-based fuel economy testing/simulation. (Of course, can't find the link at the moment...) Or rather, that the automakers took it into account when providing data to the EPA based on their test procedures.



So why then would the presumably heavier as well as less aerodynamic 2009 Jetta Sportwagen with its likely higher Cd and (slightly) more frontal area (standard roof rails) have an identical fuel consumption rating as the Jetta sedan equipped with the same drivetrain?



I find it difficult to believe that the actual fuel consumption isn't at least 2.4% different (2.4%=1 mpg from the 41 mpg hwy rating for the TDI M-6) between these two vehicles.

Yet the cars' ratings are identical for every powertrain variation.

I don't buy it.

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Old 11-06-2008, 05:23 PM   #2 (permalink)
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This has been brought up a few times. Either the EPA doesn't account for aero, or it doesn't account for it enough to make a noticable difference. I'd like a solid answer on this too.
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Old 11-06-2008, 05:50 PM   #3 (permalink)
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Since they can't test every vehicle, they test similar engine/transmission combos of similar models and make an "Estimate" -- despite the added weight and different aero of the wagon.

Attached is the raw data file for '09 as a reference (several years can be downloaded directly from the EPA)...

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Old 11-06-2008, 06:07 PM   #4 (permalink)
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What extra weight?
2009 Volkswagen Jetta wagon = 3228 lbs
2009 Volkswagen Jetta sedan = 3230 lbs
2009 Volkswagen Jetta specs, auto safety at Edmunds
2009 Volkswagen Jetta specs, auto safety at Edmunds

Drag Coefficient of .31 for both Jetta sedan and wagon.
http://www.cowellvw.com/jettawagonfeatures.pdf
Same thing happend with most Volvo sedans and wagons. There is essentially no aerodynamic drag penalty for the wagon.

Which is pretty pathetic considering the 1998 Passat had a CD of .27 for the sedan and .28 for the wagon. Even a 1994 Volvo 850 wagon had a CD of .31. So much for progress.
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Old 11-06-2008, 06:10 PM   #5 (permalink)
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Nothing new. They did that for my 20 yr old cars too. They rated the sedan and wagon the same mpg, even though the wagon is about 300-400 lbs more and every one of these wagons Ive seen has a roof rack. They also rated the Pontiac 1 mpg better than the Chevy, even though they are identical powertrains, electronics, and 99% identical sheetmetal.
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Old 11-06-2008, 06:17 PM   #6 (permalink)
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Quote:
Originally Posted by wagonman76 View Post
Nothing new. They did that for my 20 yr old cars too. They rated the sedan and wagon the same mpg, even though the wagon is about 300-400 lbs more and every one of these wagons Ive seen has a roof rack. They also rated the Pontiac 1 mpg better than the Chevy, even though they are identical powertrains, electronics, and 99% identical sheetmetal.
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.
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Old 11-06-2008, 07:42 PM   #7 (permalink)
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Round off error in published figures?

for example...
Sedan has a cD of .305
Wagon has a cD of .314

So published figure is .31, for both - despite the 9 point difference?

That still doesn't explain EPA figures - I would hope they use at least three decimal places... And there's no excuse for bigger FA

---
Quote:
Since they can't test every vehicle, they test similar engine/transmission combos of similar models and make an "Estimate" -- despite the added weight and different aero of the wagon.
Or that makes sense
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Old 11-06-2008, 07:57 PM   #8 (permalink)
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Quote:
Originally Posted by tjts1 View Post
What extra weight?
2009 Volkswagen Jetta wagon = 3228 lbs
2009 Volkswagen Jetta sedan = 3230 lbs
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.
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Old 11-06-2008, 09:58 PM   #9 (permalink)
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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!
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Old 11-06-2008, 10:05 PM   #10 (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.)
Yeah, same here -- I made the same mistake. Older wagons traditionally weighed more.

-Rick

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