Diesel mpg underrated on the window sticker

Many diesel drivers know that the window sticker EPA figures under report what the vehicle can get. Apparently, the EPA also knows this also.

http://www.epa.gov/carlabel/documents/420r06017.pdf

The PDF is on the study the EPA did for the label and testing changes for fuel economy testing in 2008. There is a nice little table on page 16, but I don't how to neatly cut and paste it to a post. So here is a summary of the data.

The data comes from the Oak Ridge National laboratory Your MPG program. It is a collection of user reported data. It appears to have been rolled into the fueleconomy.gov site as the user mpg number under car entries. The MPG-based label where diesels did 18% better became the mathematically adjusted one used in 2008.

As with other user reported sites, like Fuelly, there is selection bias. Only the people interested in mpg are reporting, and they are likely driving more efficiently and/or have a more efficient model. This would be of all the cars reported in the data, not just the diesels. So while the average diesel driver may not be getting that 18% difference, they are likely getter mpg in comparison to the label than gasoline drivers. An assumption, but a reasonable one.

Not diesel specific, but of some interest:

MOVES is a modeling system of vehicle emissions. MOVES (Motor Vehicle Emission Simulator) | Modeling and Inventories | US EPA

So the EPA uses 43/57 city/highway split for one program, and this may be what actual drivers experience, but uses 55/45 for the combined number on the window sticker and website. I'm not sure how the "embedded in the 5-cycle formula" works into this. It may not even hold true today and have changed seeing how this paper was written before implementation.

Edit: adding a current link for the paper. Document Display | NEPIS | US EPA

 

It isn't a comparison between drive types, but a comparison of the types to their EPA window sticker numbers. Because diesels do better than their labels doesn't imply that they do as well as hybrids.

The people submitting their data to Oak Ridge were the ones stating a 43/57 split in their drives on average. At least those that filled in that section did.

 

The average of the two 328ds on fuelly is 14% higher than combined. The Golf and Jetta are 14% to 15%. The Passat is 17% to 18% higher.

The Passat has about the same cabin space as the Hyundai Sonata hybrid, with a lot more trunk space. The numbers at fuelly have the Passat doing better.
Compare Side-by-Side

The price difference here is only around 15%. The cost of USLD is a hurdle for many in getting a diesel. But not everyone wants a hybrid, or even have one as an option in the car segment they are looking at, or they actually do drive most of the time on the highway. Choice is good.

 

What year was the Rabbit? IFAIK, the only diesel ones were from the '80s. It may not have been geared for today's highway speeds for best efficient. I do know they didn't have a turbo, which improves a diesels efficiency and performance.

Under the revised EPA numbers, the 4-spd manual had a 38mpg combined, the 5-spd got 36, and the 3-spd automatic got 29.
Compare Side-by-Side

 

I learned to drive manual in my friend's 1980 diesel Rabbit.
It had the electrical issues.
It is comparatively smaller, and likely lighter, than modern cars in its class.
I'm sure there were mpg numbers, but the fueleconomy.gov site only goes back to 1984.The original numbers for the 84 were 40/44 city/hwy. The adjusted are 33/40.
Depending on your driving mix(I'm guessing mostly city with your c numbers), 40mpg was good for the car.

 

Which just illustrates what the study says about the window sticker numbers underrating a diesel. The Prius is rated 50mpg. So the real world number is 4.2% lower than EPA. Regardless of transmission type, the Jetta is 34mpg combined on the sticker. Which means the real world numbers are 33% higher than the published EPA rating.

So, with all the bother the gen2 Prius and original Insight caused earlier, and the Ford hybrids recently, leading to testing and reporting changes in the recent past and possibly in the future, why don't we also adjust the procedures to more accurately reflect what diesels get in the real world?

 

The EPA's study pegged the underrating at 18% average. This isn't about giving diesels some unwarranted advantage. Just eliminating the disadvantage.