Thanks for the experience,Hi Andy,
Actually a tonneau cover increases drag. I spent 10 hours in the Lockheed wind tunnel on several occasions trying to improve Race Truck aerodynamics and the tonneau was top on my list of things to try. Results was a significant increase to the tune of about 10% in aero drag for a tonneau cover. Ditto for tail gate removal... Best increase was a 6" strip across the box at the tailgate. Doing the smoke test you could see why this worked when other items did not work....Air flow is not intuitive and you cannot see its flow. For the cab the air rolls and spirals in such a fashion that this 6" strip alters the spiraling. It was amazing that the Ford Aerodynamist with me could envision such things working...
So I wish to debunk this theory that tonneau covers on pickup trucks decrease aero drag, Tonneau covers increase drag...
Ranger Vehicle Engineering
Ford Motor Co. Retired
I do think it varies from truck to truck, or when towing the effect may be different.
On my former Ranger, we did an unscientific experiment.
With no cover, I placed a paper cup inside the box next to the tailgate, at around 60MPH the cup would suddenly slam into the front of the box from the air coming over the cab and dropping into the box right about the tailgate and causing a low pressure area behind the cab.
When I raised my sunroof (about3") the cup would stay calmly at the tailgate all the way to the truck's rev limiter.
I don't know if there is more or less turbulence with the box cover installed and the sun roof down,but I do know that the sunroof pushed the air beyond the box when it was up. That would then seem to mean a zero effect good or bad for the box cover in those particular conditions.
I just installed a box cover for an upcoming trip which also includes a travel trailer so I was curious about the effect.
I doubt that the difference will be significant enough to be easily measurable, especially since the box will be full of boxes and gear either way.
As you say, it is not all it seems, My 1976 Econoline had a drag coefficient of .33 whereas the average bicyclist is about a 1.0.
I have also found that bicycles in an open trailer produce notably better fuel economy for the tow vehicle than bicycles on the roof without a trailer.
Most all of this is anecdotal and the purpose of a box cover is to secure cargo and keep it safe,dry, and out of sight