I think this is the specific bit that you are referencing;
CAFE has separate standards for "passenger cars" and "light trucks" even if the majority of "light trucks" are being used as passenger vehicles. The market share of "light trucks" grew steadily from 9.7% in 1979 to 47% in 2001, remained in 50% numbers up to 2011.
Interesting also;
In addition, a Gas Guzzler Tax is levied on individual passenger car models (but not trucks, vans, minivans, or SUVs) that get less than 22.5 miles per US gallon (10.5 L/100 km).