Why hasn't anyone released a HD truck with a Turbo V8?

Which brings us to the aspect of CAFE. I will admit ignorance on this (big surprise), so enlighten me. Is it true that the CAFE standard is an average of all the vehicles they build? i.e. If they build enough fuel efficient cars, and hybrids, they can get away with more on the trucks/SUV's? Or does each model have to meet certain standards? If the former, they could compensate for the trucks with enough other fuel sipping econo boxes. Or am I way off base here?