Tonneau Cover
Covering up the bed of your truck creates a smooth surface for air to flow over. Less drag results in higher MPG and better performance. The prices for these range pretty widely. I suggest checking out ebay or rigging up a ghetto one by yourself. They’re not particularly expensive if you don’t need a fancy one.
And no, putting down your tailgate isn’t going to help with air resistance. Mythbusters busted that one.