This is a question that has bugged me for years. I live in Eastern Montana, and the distance between towns is about the same as the latitudinal width of many eastern U.S. states. Many of us drive pickups. My question is: Will I improve my gas mileage of my pickup if I drive with my tailgate down? Less wind resistance, and all that?
Mythbusters tried this and got better gas mileage with the tailgate up. With the tailgate up, the air in the bed of the truck circulates around in a steady pattern that results in the overall air flow across the truck being smoother. With the tailgate down, you don’t have this vortex of circulating air to smooth out the airflow, so you end up with more drag and worse gas mileage.
So, there you have it. Goofy-looking mesh gate is better than other options. To improve a little bit beyond that, reduce weight. Throw out all that stuff you’re carrying around for no recent reason. Wheel covers? Ditch 'em. Jack and spare tire? Leave 'em at home. Got a bed liner, like the Mythbusters truck? Strip it out. Trailer hitch? Back seat? Passenger seat? Well, you get the picture.
If you have seen ads for weird little devices that promise to help your gas mileage, ignore them. They will lighten your wallet, but the weight of $200 won’t help your MPG.