I would be very curious to see details on different wind tunnel tests.

What I see here is a lot of examples, some contrary, some concurring, and a lot of sound reasoning. For example, the thread starter brings up what appears to be a valid water tunnel test concluding the tail gate down is better. I mean I have heard of the tailgate-up tests myself, but have also experienced positive effects of having it down. Others have too. My truck was not stock though. I think it is unreasonable to assume all who believe their trucks got better mileage with the tail gate down are deluded as it is unreasonable to assume those that see no difference are.

So what does all this mean? I think it means there's more going on here than meets the eye. Example, looking at the truck from the drivers side we have a clockwise vortex in the bed area. But, maybe air passing underneath creates an opposing vortex behind the tailgate. Do they interact? If so how? At what speed? Maybe at a given speed they annihilate one another? Maybe something entirely different is going on. I don't think there is a be all end all to this subject. I suspect differences between front/rear height, over all height, cap to bed geometry, average sustained speeds, body lifts, and who knows what else all come into play and interact.

I'm sure most of the data is not accesible but if anyone knows how to get it, I for one would love to read it.

Frank.