OK, were getting the idea, but the details are still messed up...
Cars, trucks, etc... fall into one of three categories:
Category 1: Passenger Cars - self explanatory
Category 2: Light Trucks - Pickups, SUV's, and Vans (INCLUDES minivans) under 8500 GVW
Category 3: Medium duty and up - anything over 8500 GVW - most 3/4 ton and up pickups, vans, and SUV's
Category 1: All vehicles that a manufacturer sells here must average 27 mpg overall. Miss the target = big $ penalties. Has not changed for over a decade.
Category 2: All vehicles a manufacturer sells here must average 20.7 mpg overall. Miss the target = big $ penalties. Also has not changed for a very long time. Some controversy on whether or not minivans should be included here, but for now they are classified as light trucks. PT cruiser is also in this category (don't ask why), but Chrysler needed these to balance off the trucks. Puts a new light on the mini-utes, doesn't it?
Category 3: No CAFE limits apply here due to the varying duty cycles of the engines in this class that are assumed to actually be doing real work. Emissions do still apply, but less restrictive than the under 8500 GVW category.
Personally, I don't think the CAFE limits are then end of the world. Anything new pushes out of our comfort zone (witness the backlash against 5w20 oils around here - yet we haven't had mass reports of failures to date). With CAFE limits stagnant for so long, we have (in case you've missed it) been engaged in quite a little power war. The numbers we pull out of engines these days are mind boggling, and yet we still get the fuel mileage too. Perhaps its time to bump the CAFE numbers a bit to promote some more thought on the fuel economy side instead of the power side?