Pickups are designed so that airflow is routed from the top of the cab to the top of the tailgate, at best a topper probably gives you no effect on fuel mileage, same with tonneu covers. Dropping your tailgate disrupts the aerodynamics because it turns the entire bed of your truck into a single huge low pressure vortex generator and breaks the laminar flow designed with it up. It's pretty much guaranteed to hurt highway mileage.
They tested this on mythbusters, and what you said is exactly true. Tailgate up gets better gas mileage than tailgate down. They called some truck manufacturer, I think it was GM, and that company confirmed that tailgate up got better gas mileage in the company tests as well.
The trick to aerodynamics is that it's not the front that matters so much, but the back. To have sharp right angles at the back would generate a huge vacuum. Truck canopies tend to have a straight back end with no ducktail, so it creates a huge vacuum and messes up your gas mileage. The fuel economy of a truck could be significantly improved by putting a ducktail on it, but it would look so gay that nobody would want it.
Good air flow looks silly. Can you imagine a pickup truck having a gradual sloaping back like that? It would basically look like a minivan.
I wonder if the domestics will every take a hint from Toyota and just use a synthetic bed. I love the one in the current generation Tacoma. Bedliner, line-x, what's that?
Most trucks have it as an available option. Even cars have that as an option. If I really wanted, I could pay an extra $100 or whatever and have a hard plastic lining in the trunk of my corolla. Serious.
i have no idea why girls think they should have trucks when they have no intention of using them like trucks should be used.
-RWD instead of wrong wheel drive
-usually higher up and better in snow than a minivan
-stock all season truck or SUV tires with deep treads work really good in winter whereas all season minivan tires are total shit
-a fifth wheel camping trailer can be connected to a truck
-you can park in deep snow without getting stuck and calling your boyfriend to help you out
-some trucks come with 120V AC plugs built in so you can use a curling iron while driving to work