Does a tourneau cover on a pickup truck increase gas mileage?

yes, a tonneau cove helps a little, it decreases the amount of wind drag on your tailgate, but so does taking the tailgate off or just putting it down if youre going on a long trip

The previous answer given is technically incorrect. Usually tailgate down you increase the drag on a truck and will get worse mileage. A cover will actually improve you mileage a little but the weight of the cover may negate a portion of this. There are several studies cited online for this. Most places that talk about lowering the gate aren't basing it on science but on what they think is logical. Aerodynamics isn't about logic, it is a very complex science. Mythbusters actually tested this out, do a search on it.