Garrett Graf commented "How much fuel you would save driving on 4 small spare tires?"
Does anyone know who tried that?
People keep mentioning tailgate up, tailgate down, and mesh instead of a tailgate. I found the clip, which will undoubtedly die eventually, but they said the mesh was 5% more efficient than tailgate up or a tonneau cover:
Jamie said that instead of e-mailing them with any further fuel efficiency questions, "Just drive the way you want."