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.

http://blogs.thecarconnection.com/im...k/tmb/7052.jpghttp://ecomodder.com/forum/attachmen...1&d=1481278704

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?

http://ecomodder.com/forum/attachmen...1&d=1481278878

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.

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.

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)...

-Rick

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.

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.

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

---
Or that makes sense :p

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'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!

Yeah, same here :o -- I made the same mistake. Older wagons traditionally weighed more.

-Rick

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?

No source, pure speculation for an example of the large range contained within round off error.


Until it makes an acute angle change to make the back portion of the car o.0

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...

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).

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?

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.

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

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.