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Sometimes, when science and technology issues have economic or political consequences, incorrect or misleading information and analyses can be propagated through the media, leading both decision makers and the public astray.
A piece published in April by Forbes, covering the effects of the 2022 through 2025 corporate average fuel economy standards, is a case in point.
With the news that the NHTSA is now considering not just freezing vehicle-emission standards at 2021 levels, but possibly rolling them back to earlier and lower figures, proper analysis and understanding becomes even more important.
DON'T MISS: NHTSA to review gas-mileage rules: could freeze 2021 limits, even roll back (updated)
The gist of the Forbes article was to downplay the importance of the CAFE rules for 2022 through 2025.
"According to the EPA’s own estimates," wrote author Sam Ori, "the overwhelming majority—nearly 90 percent—of originally projected fuel savings under the current policy will have been locked into place by 2021" (my emphasis).
But the EPA said no such thing, and it did not provide that estimate. (An explanation of the mechanics of Ori's calculation is included at the end of this article.)
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The statement seems to imply that 90 percent of the consumption reduction occurs between the years 2012 and 2021 (the first of two phases of emissions reductions from 2012 through 2025), while only 10 percent occurs during the second phase, from 2022 to 2025.
To readers who interpreted the statement in that way, it's wildly misleading. So let’s set the record straight.
First, CAFE regulations are, well, complicated. There are separate rules for cars versus trucks. The requirements depend on the size (footprint) of the vehicle.
Window-sticker fuel economy on the vehicle is "adjusted" through a variety of calculations to reflect on-the-road reality, but CAFE rules work off unadjusted numbers taken directly from the fuel consumed in emission testing.
There is also the non-linear nature of MPG, which can be confusing and misleading itself.
This article tries to keep the jargon to a minimum, to focus on the significant impact the 2022-2025 rules will have on reducing fuel consumption, and the resulting pollutants emitted by that fuel consumption.
Figure 1: Passenger-car fuel consumption under CAFE standards by year [graph: John Briggs]
Figure 1 shows the CAFE standards for cars, and how they must improve over the years from 2012 through 2025.
The vertical axis is in units of consumption (gallons per 1,000 miles) to eliminate the non-linearity issues associated with miles per gallon, its inverse.
There are three lines for different sizes of vehicles, showing footprints of 40, 47, and 56 square feet, which roughly correspond to a Honda Fit subcompact hatchback, a Honda Accord mid-size sedan, and Mercedes S-class large luxury sedan in size.
CAFE allows larger vehicles to consume more fuel than smaller ones, allowing buyers to select from various sizes and classes of vehicles.
The rate of decrease in consumption is more or less linear with time: roughly 70 percent of the decrease occurs from 2012 through 2021, while the remaining 30 percent occurs from 2022 through 2025.
So rather than the 90-percent/10-percent split implied by Ori, the 2012-2021 and 2022-2025 CAFE rules are split 70 to 30. But that is just the start of the story.
Figure 2: Light-truck fuel consumption under CAFE standards by year [graph: John Briggs]
Figure 2 shows the same data, this time for light trucks. Three curves are shown for different vehicles, with footprints of 40, 55, and 75 square feet, which roughly represent a Honda HR-V compact crossover, a Honda Pilot mid-size crossover, and Ford F150 SuperCab full-size pickup truck with a 6.5-foot bed.
The highly non-linear nature of the last (green) curve is most interesting.
Curiously, the EPA and the NHTSA have each given the largest trucks a pass from 2017 to 2021, and require almost no improvement in fuel efficiency during this period.