It's been a rough time for the official fuel economy figures for the Ford C-Max Hybrid. When the car was released in 2012, Ford made a huge deal about how it would beat the Toyota Prius V, which was rated at 42 combined miles per gallon, 44 city and 40 highway. The Ford? 47 mpg across the board.

How did Ford come to this place, where its Prius-beater turned into an also-ran?

Well, after hearing customer complaints and issuing a software update in mid-2013, then discovering a real problem with the numbers last fall and then making a big announcement last week that the fuel economy ratings of six different 2013 and 2014 model year vehicles would need to be lowered, the C-Max Hybrid has ended up at 40 combined, 42 city and 37 highway. In other words, the Prius trumps it, as daily drivers of those two vehicles have known for a long time. The changes will not only affect the window sticker, but also the effect that the C-Max Hybrid (and the five other Ford vehicles that had their fuel economy figures lowered last week) have on Ford's compliance with greenhouse gas and CAFE rules for model year 2013 and 2014.

How did Ford come to this place, where its Prius-beater turned into an also-ran? There are two technical answers to that question, which we've got below, as well as some context for how Ford's mistakes will play out in the bigger world of green vehicles.

2013 Ford C-Max Hybrid

Let's start with Ford's second error, which is easy to do since we documented it in detail last year (the first, needing to do a software update, was also covered). The basic gist is that Ford used the general label rule (completely legally) to test the Fusion Hybrid and use those numbers to figure out how efficient the C-Max Hybrid is. That turned out to be a mistake, since the two vehicles are different enough that their numbers were not comparable, despite having the same engine, transmission and test weight, as the rules require. You can read more details here.

Ford's Said Deep admitted that the TRLHP issue is completely separate from the general label error from last year.

Now let's move on to last week's announcement. What's interesting is that the new recalculation of the MPG numbers – downward, of course – was caused by a completely separate issue, something called the Total Road Load Horsepower (TRLHP). Ford's Said Deep admitted to AutoblogGreen that the TRLHP issue had nothing to do with the general label error from last year. "When we retested [the C-MAX Hybrid] last summer, we had not yet run the production vehicle validation tests that revealed the error related to how we correlate wind tunnel results into the TRLHP model," he said.

So, what is that model? Well, it's not easy to understand, but two detailed places to dig in are here and here (PDF). Ford's announcement yesterday said that:

TRLHP is a vehicle-specific resistance level used in vehicle dynamometer testing that determines fuel economy ratings. TRLHP is established through engineering models that are validated through vehicle testing, including physical track tests referred to as coastdown testing.

The EPA described TRLHP to AutoblogGreen this way:

When using the dynamometer testing, certain factors/inputs need to be accounted for so that the test is as accurate as possible in reflecting real-world conditions. One of these factors is TRLHP. The TRLHP is derived from an engineering model and is validated from vehicle track testing. To calculate it, one must account for different variables such as crosswinds, aerodynamics, and tire friction.

In Ford's case, it wasn't just the TRLHP number that was off. Raj Nair, Ford's fall guy for hybrid mpg overstatement, told reporters that, "we do a physical wind tunnel test, and then use a correlation factor to enter that into the engineering model – which is then the total load horsepower test. The error was in the correlation factor in the model."

2014 ford fiesta

It's not only the C-Max Hybrid that was downgraded yesterday. The C-Max Energi, Fusion Hybrid and Energi, the Lincoln MKZ and most versions of the 2014 Fiesta were also acknowledged to be less efficient than previously stated. Despite all of these revisions – plus the Hyundai/Kia mpg errors – the EPA told AutoblogGreen that it doesn't expect that we'll see more changes in the future:
We generate over 1200 labels each and every year, and these three re-labelings represent a tiny fraction of that amount. With the majority of consumers achieving or bettering the label fuel economy estimates, based on consumer data provided to and other websites, we are confident with vehicle test results and fuel economy label values.
Despite the wording of the EPA's press release that made it sound like the government agency pressured the automaker to change the MPG rating, the EPA confirmed that Ford came to the government with the news:

On March 28, Ford alerted EPA that they had identified a possible error affecting their fuel economy values. EPA immediately began working with Ford on an extensive re-testing program to correct the error, overseeing Ford's fuel economy tests, and also conducted independent testing at our Ann Arbor lab to confirm Ford's results. Under EPA regulations, Ford is required to correct fuel economy labels on affected vehicles within fifteen days.

Electric vehicle supporter and star of Who Killed The Electric Car? Chelsea Sexton said that Ford deserves credit for the, "the straightforward announcement, apology, and goodwill payments" and that the real interesting issue will be, "how the increased focus on these numbers affect EV range claims used by automakers in marketing, if not on Monroney stickers. Given the consequence to new drivers and, ultimately, the success of EVs, overly optimistic range numbers are the perfect example of 'just because you can, doesn't mean you should'."

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