Complexifying the report on the Fusion and C-Max hybrids a little

First Hyundai and Kia, now Ford? Last week Consumer Reports questioned the official estimates of fuel economy for Ford’s new Fusion and C-Max hybrids, reporting that in their tests “the Fusion Hybrid delivered 39 mpg overall” while the C-Max Hybrid came in at 37 mpg, neither of which is very close to the 47 combined mpg fuel economy label. In its story CR notes a little darkly that “automakers mostly self-certify their cars. Then, the EPA spot-checks about 15 percent of them with its own tests in a lab. We have reported our fuel-economy results to the EPA.” Now the EPA is going to look into the matter.

I commented previously on what the Hyundai/Kia episode illustrates about compliance and enforcement programs, which are a crucial element in any vehicle efficiency regulatory scheme. But there are some additional aspects to this situation with the Fusion and C-Max hybrids that are worth calling to light.

Consumer Reports notes that “EPA fuel-economy estimates are the result of testing on a dynamometer” while their test measures “real-world fuel economy.” For all the merits of that approach, it does have some disadvantages that need to be taken into account. “Real-world” tests are, unsurprisingly, subject to real world variation, and for that reason are not as useful for comparison between vehicles. Consumer Reports tests its vehicles year round and outdoors, in New York. Temperature has a major impact on vehicle fuel economy, so this is a major problem when comparing different vehicles tested at different times of the year. CR does apply a temperature correction factor, but any temperature correction factor is woefully inadequate, as temperature impacts vary widely from vehicle to vehicle. Nor can vehicle speed and acceleration be controlled nearly as well on a test track as on the dyno test used by EPA, which means additional variation in test results.

The second point is that fuel economy (mpg) is the wrong metric, as it is non-linear. Gallons per mile is the linear metric; miles per gallong is the inverse relationship and mpg explodes as gallons per mile approaches zero, as my colleague Anup Bandivadekar pointed out here. In the case of the C-max, the difference between 37 mpg and 47 mpg is 0.57 gallons per 100 miles. The difference between 20 mpg and 22.5 mpg is also 0.57 gallons per 100 miles. Thus, the differences between the CR and EPA test results are exaggerated by the use of mpg for these very high-efficiency vehicles.

Third, CR’s test procedures gives a little more weight to highway driving than EPA’s does. This means that vehicles with very high highway ratings, such as diesels, tend to do better on CR’s procedures and vehicles with very high city ratings (i.e., hybrids) tend to do better on EPA’s procedures. In this case, its not a question of whether one is better than the other. Both are representative, just of different drivers. Examples:

Model CR mpg EPA mpg gal/100 mile difference
2010 VW Golf diesel 38 34 (0.31)
2012 Civic Hybrid 40 44 0.23
2011 Sonota hybrid 33 36 0.25
2006 Civic Hybrid 36 42 0.40
2010 Prius 44 50 0.27

Having said all of this, the gap noted by CR on the new C-max does seem to be higher than has been seen on previous hybrids, even in gal/100 mile space. Here’s some relevant comparative data from, where 23 owners have self-reported their fuel economy with the C-max hybrid and 16 have reported their fuel economy on the new Prius V, which is similar in configuration to the C-max hybrid.

Model # reports Avg mpg EPAmpg
Prius V 16 43.0 42
C-max 23 39.5 47

The very limited data from tends to support the CR findings on the C-max. The high interest from consumers in fuel economy and the aggressive fuel economy standards that are being rolled out both increase the incentive for manufacturers to find ways to improve their efficiency ratings—and to push the boundaries of the test procedures. Fortunately, the EPA has extensive enforcement authority and procedures and is investigating.

Vehicle testing