Tailgates and wind resistance

Which gets the best mileage for my pickup: Tailgate up, tailgate off with a cargo net, tailgate up with the camper shell on?
('91 Ranger with stick, small 2.3 liter engine, mostly highway driving.)

Thanks, troub. Very informative.

Oh, I thought this was just the standard tailgate question. I see you mentioned a shell. Assuming the camper shell is level with the roof of your truck, I would think that “tailgate up with camper shell on” would be fairly similar to just plain “tailgate up,” considering the stagnant air pocket in the bed that deflects air over the tailgate to the back of the vehicle. Instead of the air pocket, you’ve got the camper shell. The camper shell might actually be more turbulent, though (you don’t see many sports cars with big flat square rear ends?).