The thing that sucks is say a company makes two cars, one that gets 20mpg and one that gets 100mpg, while the average for the company is 60mpg, that is not the same as the CAFE required average.
CAFE uses the “harmonic” mean, not the simple arithmetic mean.
The harmonic mean is measured:
X / (1/Y1 +1/Y2) = Z
X = Number of cars in fleet (2 in my example)
Y = MPG of a car, continuing for each vehicle in the fleet (Y1 = 20, Y2 = 100)
Z = CAFE measured average MPG across the fleet.
So by my calculations, CAFE would measure it as 33.33mpg across the fleet, but using the math we learned through school, it is 60mpg. Pretty fucked up IMO.