The C-Max MPG Fix: Getting it Right is Good for Consumers and Automakers Alike

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The recent announcement from Ford that they will be changing the MPG rating of their Ford C-Max hybrid isn’t just a reminder of how much consumers care about fuel economy, but how important it is to get the ratings right. Accurate ratings can be a challenge as vehicles get more efficient since small variations in fuel consumption are amplified in the vehicles’ MPG ratings. The experience with the C-Max is moving the EPA to reexamine some aspects of their MPG labeling certification requirements to make sure they keep pace with advances in fuel economy.  This is good news for both automakers—who tout vehicle ratings as key selling points—and consumers, who depend on MPG ratings when comparing their vehicle choices.

Small changes in fuel consumption are amplified in higher MPG vehicles

Getting the fuel economy ratings to closely represent real world driving has always been a challenge.  People have different driving habits (example: jack rabbit starts versus steady acceleration), use their cars differently (example: highway driving versus stop-and-go traffic or carrying heavy cargo), and operate in different environments (example: Minnesota winters versus Florida summers).  The EPA has made changes to how they factor in these different aspects of vehicle performance and use over the years to arrive at an average MPG estimate that reflects “typical” driving.  But as they say, your mileage may vary.


Ford recently lowered the MPG rating on the 2013 C-Max from a combined city/hwy rating of 47 mpg to 43 mpg. Photo: Flickr user mariordo59.

As cars become more efficient and use less fuel, small variations in fuel consumption are amplified in the MPG ratings of the most efficient vehicles.

Consider two vehicles, one that achieves a 20 mpg rating (consumes 5 gallons for every 100 miles driven) and one with a 50 mpg rating (consumes 2 gallons for every 100 miles driven).   What happens to the actual miles per gallon of these vehicles if the fuel consumption of these vehicles varies by plus or minus ½ gallon per 100 miles?

The 20 MPG rated vehicle would achieve 18 to 22 miles per gallon on the road – a range of only 4 mpg. The 50 mpg rated vehicle however would achieve 40 to 67 miles per gallon on the road—a whopping 27 mpg range.  That big difference in MPG is one that is pretty hard to ignore.

This example exaggerates the issue, but illustrates one reason why higher MPG vehicles are getting a lot of scrutiny by consumers.   In the case of the C-Max, the rating was reduced 4 mpg, from 47 mpg to 43 mpg, the difference of 0.2 gallons per 100 miles.  Apply that difference in fuel consumption to a 20 MPG vehicle and the rating would have dropped by only 1 mpg.

Getting MPG right: What’s at stake?

Automakers use vehicle MPG ratings as a key selling point to distinguish themselves from the competition and because consumers value fuel economy in their purchasing decisions.  When ratings don’t match performance,  it’s not just bad for consumers who aren’t achieving the fuel savings they expect, but it’s bad for business.  The  voluntary MPG rating downgrade of the C-Max also led Ford to offer owners a rebate for fuel savings they were promised but didn’t achieve.  Hyundai and Kia offered a similar financial reimbursement to its customers when fuel economy ratings had to be corrected and Honda faced a class action lawsuit and was taken to small claims court over the fuel economy ratings of an earlier model of the Civic hybrid. The underlying circumstances of each of these examples vary widely, but they highlight the importance of getting MPG right.

In the case of the C-Max, Ford utilized a flexibility built into the compliance options for EPA labeling requirements to arrive at the MPG rating. With an increasing number of high efficiency vehicles showing up in automaker show rooms, that flexibility may no longer be appropriate and the EPA is planning to revise the requirements.  The process for developing MPG ratings needs to keep pace with technology development. Both consumers and auto manufacturers stand to benefit.

Posted in: Vehicles Tags: , , ,

About the author: Don Anair is a senior engineer with expertise on diesel, hybrid and battery electric vehicle, and goods movement technologies and the policies needed to turn them into real solutions for U.S. oil dependence, air pollution and global warming. He holds a master’s degree in electrical engineering. See Don's full bio.

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  • Dan5


    It appears from the crowd sources ( as well as john voelcker in green car reports that the Fusion and Cmax vary more widely than the other hybrids (out of 11 hybrid cars, only 3 obtained significantly less mpg than advertised; those cars were the Fusion, Cmax and Lexus 450h).

    Since you are an expert in diesel, hybrid and BEVs. I would like your opinion in how you think the Fusion/CMax threshold played a role in the mpg claims. Ford makes the claim that the Fusion/CMax can operate up to 62 mph in pure electric mode, and most of the test is spent below 62 mph. Do you think this would have any bearing on skewing the results?

    Thank you

    • Don Anair

      I haven’t investigated how the threshold might affect the variability in fuel economy performance in the real world versus on the test, so can’t speak to your specific question.

      The particular issue related to the downgrade in the C-Max MPG rating is a compliance provision in the EPA rules that allows one model’s MPG test to be used for another model – if they have the same engine, transmission, and test weight.

      • Dan5

        I do have another question that you may be able to answer too. I am very curious about how the Fusion obtains such great mpg 47/47 compared to a Prius 48 highway/51 city; average 49.5

        I’ve tabulated some of the key factors that effect fuel economy for you

        Fusion (hybrid) Prius
        weight 3615 3042 lbs
        CoD 0.27 0.25
        battery capacity 1.4 kwhr? 1.2 kwhr
        horsepower 188 134
        acceleration (0-60) 9.3 9.8
        engine efficiency 37% 38.5%

        I am curious about how that only correlates to a 2.5 mpg difference. To me everything looks far worse for the Fusion, and one would expect it to get much worse mpg than a Prius. 600 extra lbs, more wind resistance, and a more powerful, less efficient engine tend to tell me it should get significantly less than a Prius.

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