Of hybrids, metrics, and C-Maxes


The recent spate of news on Ford lowering the mileage rating on its C-Max hybrid (e.g., here and here and here) could lead some to question the in-use fuel economy benefits of hybrids. That would be a mistake, because the criticism leveled at the C-Max is arguably inaccurate; the discrepancies between label and actual fuel economy in that case can be traced pretty clearly to arcane details of calculating fuel economy that are not widely known or understood.

The measurement of fuel economy on the test cycles is actually pretty good, and accurately reflects the efficiency of the vehicle tested—on the test cycles. EPA and are clear that the primary purpose of fuel economy labels are to facilitate comparison with other vehicles. For this purpose, standardized testing is highly desirable. But, as famously noted on the fuel economy labels, “your mileage may vary.” That’s especially true of hybrids, because they are more sensitive to how they are driven and ambient conditions than conventional vehicles. The variation in in-use fuel economy for hybrids is larger than for conventional vehicles. Some people exceed the label values. Some don’t come close. It’s a function of how the vehicle is driven (for example, lots of short trips are really bad for efficiency) and ambient conditions.

Another issue is that for purposes of calculating FE label values, vehicles with the same engine, transmission, and weight class are grouped together. That practice was codified by the EPA decades ago to reduce the burden of testing vehicles on manufacturers. There are so many variants of similar vehicles that requiring each variant to be tested would impose huge costs on manufacturers, and would produce relatively little impact on FE label values in return.

The procedures are codified in 40 CFR Part 600. Before calculating FE labels for a given model, models are grouped into “base levels.” The definition of a base level is just engine type, transmission type, and inertia weight class.

  • All vehicles with the same engine type, transmission type, and inertia weight class are grouped together, and a single FE value is calculated for each base level.
  • The vehicle within each base level with the highest volume of sales must be tested. Manufacturers can voluntarily test more vehicles within a base level, but are not required to do so.
  • If more than one vehicle within a base level is tested, the results are harmonically averaged using a sales-weighted average.
  • This calculated number for the base level is directly applied to all model types that have the same engine type, transmission type, and inertia weight class. So, all of the similar model types always get exactly the same FE value.

Ford was simply following this codified procedures. And EPA has decided that it needs to change the fuel economy label procedures for hybrids and Ford has voluntarily agreed to do this now.

The real message here is that miles per gallon is the wrong metric. In the case of hybrids, the small impact on fuel-economy label values introduced by the “base level” method is artificially magnified because of the use of MPG instead of fuel consumption (i.e., gallons per mile). MPG is the inverse relationship of fuel consumption and, like all inverse relationships, it explodes as you approach low values of fuel consumption. That reduction in fuel economy from 47 to 43 mpg on the C-Max? That works out to a difference of 0.2 gallons every 100 miles (100 miles divided by 43 mpg minus 100 miles divided by 47 mpg). Really doesn’t mean much. To put it into context, a vehicle that gets 22 mpg instead of a rated label value of 23 mpg has the exact same 0.2 gallon per 100 mile shortfall. If the US appropriately rated vehicles in terms of fuel consumption, instead of MPG, there wouldn’t be any complaints about hybrid fuel consumption.